(CNN) — A top Ukrainian army officer said a “full-scale invasion” of his country was under way Thursday, as a U.S. official said up to 1,000 Russian troops had crossed Ukraine’s southern border to fight alongside pro-Russian rebels.
U.S. officials said Russian troops were directly involved in the latest fighting, despite Moscow’s denials.
Rebels backed by Russian tanks and armored personnel carriers fought Ukrainian forces on two fronts Thursday: southeast of rebel-held Donetsk, and along the nation’s southern coast in the town of Novoazovsk, about 12 miles (20 km) from the Russian border, according to Mykhailo Lysenko, the deputy commander of the Ukrainian Donbas battalion.
“This is a full-scale invasion,” Lysenko said, referring to the fighting in the south.
Intelligence now indicates that up to 1,000 Russian troops have moved into southern Ukraine with heavy weapons and are fighting there, a U.S. official told CNN Thursday.
Ukraine’s National Defense and Security Council said that Russian forces were in full control of Novoazovsk as of Wednesday afternoon.
Russia’s military fired Grad rockets into the town and its suburbs before sending in two convoys of tanks and armored personnel carriers from Russia’s Rostov region, it said in a statement
“Ukrainian troops were ordered to pull out to save their lives. By late afternoon both Russian convoys had entered the town. Ukraine is now fortifying nearby Mariupol to the west,” the NDSC said.
A number of villages in the Novoazovsk, Starobeshiv and Amvrosiiv districts were also seized, it said.
The NDSC also warned that a rebel counterattack is expected in the area where Malaysia Airlines Flight 17 was shot down in July. Ukrainian and Western officials believe it was downed by rebels armed with Russian-made weapons.
Novoazovsk is strategically important because it lies on the main road leading from the Russian border to Ukraine’s Crimea region, which Russia annexed in March. Separatist leaders in the Donetsk and Luhansk regions then declared independence from Kiev.
U.N. Security Council to meet
As international concern mounted over the apparent escalation in fighting, Lithuania requested an emergency U.N. Security Council meeting on Ukraine.
UK ambassador to the United Nations Mark Lyall Grant said Russia would be asked to explain why its soldiers are in Ukraine.
Ukrainian President Petro Poroshenko and Prime Minister Arseniy Yatsenyuk had earlier called for the U.N. meeting, as well as action by Europe.
The latest flare-up comes despite a meeting between Poroshenko and Russian President Vladimir Putin in Belarus on Tuesday at which some progress appeared to have been made toward finding a diplomatic solution to the crisis.
Poroshenko canceled a planned trip to Turkey on Thursday “due to sharp aggravation of the situation in Donetsk region … as Russian troops were brought into Ukraine,” a statement from his office said.
In a Cabinet meeting, Yatsenyuk said Russia “has very much increased its military presence in Ukraine” and that tougher measures may be needed to curb Russia’s support for the rebels.
“Unfortunately, the sanctions were unhelpful as to de-escalating the situation in Ukraine,” he said, referring to the economic sanctions imposed by the United States and European Union against Russian individuals and companies.
Yatsenyuk suggested one way to halt “Russian aggression” could be to freeze all assets and ban all Russian bank transactions until Russia “pulls out all its military, equipment and agents” from Ukraine.
“Vladimir Putin has purposely started a war in Europe. It is impossible to hide from the fact,” he said.
U.S. ambassador: Russia is directly involved
U.S. Ambassador to Ukraine Geoffrey Pyatt also said Thursday that Russian soldiers were directly involved in the fighting, alongside the pro-Russia rebels.
“Russian-supplied tanks, armored vehicles, artillery and multiple rocket launchers have been insufficient to defeat Ukraine’s armed forces, so now an increasing number of Russian troops are intervening directly in the fighting on Ukrainian territory,” he said on Twitter.
“Russia has also sent its newest air defense systems including the SA-22 into eastern Ukraine and is now directly involved in the fighting.”
Moscow denies supporting and arming the pro-Russia rebels. It has also repeatedly denied allegations by Kiev that it has sent troops over the border.
A Russian senator and the deputy head of the Committee on Defense and Security in Russia’s upper house of Parliament, Evgeny Serebrennikov, dismissed the latest reports of a Russian incursion as untrue.
“We’ve heard many statements from the government of Ukraine, which turned out to be a lie. What we can see now is just another lie,” he said to Russian state news agency RIA Novosti.
Russian lawmaker Leonid Slutsky also accused Kiev of lies, in comments to RIA Novosti.
“I can only say that there’s no ground for claims like this, and the junta tries to lay its own fault at someone else’s door,” he said, referring to the Kiev government.
Moscow regards it as illegitimate because it took charge after Ukraine’s pro-Russian President Viktor Yanukovych was ousted in February.
Rebel leader: 3,000 to 4,000 Russians in our ranks
However, the Prime Minister of the self-declared Donetsk People’s Republic, Alexander Zakharchenko, acknowledged Thursday that there are current Russian servicemen fighting in the rebels’ ranks in eastern Ukraine.
In his statement, televised on state-run Russia 24, Zakharchenko said the rebels have never concealed that many Russians are fighting with them. He said up until now there were 3,000 to 4,000 volunteers, some of whom are retired Russian servicemen.
Zakharchenko went on to reveal that the Russian servicemen currently fighting in their ranks are active, “as they came to us to struggle for our freedom instead of their vacations.”
On Tuesday, Ukraine’s Security Service said it had detained 10 Russian soldiers in Ukraine.
Russian state media cited a source in the Russian Defense Ministry as saying the soldiers had been patrolling the border and “most likely crossed by accident” at an unmarked point.
The NDSC said Thursday that Ukraine’s Security Service detained another Russian serviceman who testified that his unit was supplying heavy military equipment to militants.
Ukrainian volunteers retreat from Mariupol area
Pro-Kiev forces apparently already have engaged with rebel forces between Novoazovsk and Mariupol, the Sea of Azov port city 35 kilometers to the west that the country’s security council said was being fortified.
A CNN crew north of Mariupol saw a ragged convoy of about 25 vehicles, some with their windows smashed out, belonging to pro-Kiev volunteer fighters heading away from the city Thursday afternoon.
The volunteers, including two from the country of Georgia, said they’d been involved in fighting in the Mariupol area but didn’t provide details.
Earlier Thursday and further north, the CNN crew was near Donetsk city, which Ukrainian forces have been trying to wrest from rebels for weeks. Heavy Ukrainian artillery fire targeted areas near Donetsk’s southern suburbs amid a heavy downpour of rain.
The main highway 15 kilometers south of Donetsk was deserted. With return fire coming from Donetsk, villagers in the area said they’d been taking shelter indoors or underground, coming out only for an hour or two a day to get supplies.
U.S. State Department spokeswoman Jen Psaki on Wednesday highlighted the latest reports of heavy fighting around Novoazovsk and Donetsk airport, as well as of “additional columns of Russian tanks, multiple rocket launchers and armored vehicles” heading for communities in southeastern Ukraine.
“These incursions indicate a Russian-directed counteroffensive is likely underway in Donetsk and Luhansk,” she said. “Clearly that is of deep concern to us.”
She accused Moscow of not acting in a transparent manner when it came to the Russian people, as well as Ukraine and the rest of the world.
“We’re also concerned by the Russian government’s unwillingness to tell the truth even as its soldiers are found 30 miles inside Ukraine,” she said. “Russia is sending its young men into Ukraine but are not telling them where they’re going or telling their parents what they’re doing.”
On Wednesday, NDSC also claimed that members of a Russian tactical battalion were present in the village of Pobeda, in Ukraine’s Luhansk region.
“If these troops got lost and accidentally found themselves in Ukraine as well, they should go back East,” the update said.
The city of Luhansk, a rebel stronghold, has been at the center of fighting for days, prompting a humanitarian crisis. The NDSC said it remained without water, power or phone connections Thursday.
Journalist Victoria Butenko reported from Kiev and CNN’s Diana Magnay from eastern Ukraine, while Laura Smith-Spark wrote and reported in London. CNN’s Tim Lister, Alla Eshchenko and Ariana Williams contributed to this report.