KIEV, Ukraine (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 kilometers) from the Russian border, said 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 on Thursday.

NATO provided what it said is evidence: satellite images showing Russian troops engaged in military operations inside Ukraine.

"The images, captured in late August, depict Russian self-propelled artillery units moving in a convoy through the Ukrainian countryside and then preparing for action by establishing firing positions in the area of Krasnodon, Ukraine," NATO said in a news release.

Commercial satellite imagery shows the same, according to a British security source with detailed knowledge of UK intelligence estimates. One image that British intelligence has analyzed, dated Tuesday, shows 15 heavy trucks, at least seven armored vehicles and at least nine artillery positions.

Russia's military actions in eastern Ukraine "must cease immediately," British Prime Minister David Cameron said Thursday.

"I'm extremely concerned by mounting evidence that Russian troops have made large-scale incursions into South Eastern Ukraine, completely disregarding the sovereignty of a neighbor," Cameron said. "The international community has already warned Russia that such provocative actions would be completely unacceptable and illegal."

As the Russian presence grows, so does its influence over the separatist leadership in Ukraine, the British security source told CNN.

According to the source, the UK has determined that Russian artillery and rockets -- across the border and from within Ukraine -- have been fired against the Ukrainian military.

Two SA-22A gun/missile air defense systems were observed in separatist-controlled parts of Luhansk province on August 2, the source said. This system is not in Ukraine's inventory but is used by the Russian military.

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 say they 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 meets

As international concern mounted over the apparent escalation in fighting, the U.N. Security Council held an emergency meeting on Ukraine.

Samantha Power, the U.S. ambassador to the United Nations, accused Russia of lying.

"It has manipulated. It has obfuscated. It has outright lied. So we have learned to measure Russia by its actions and not by its words," Power said, calling for "serious negotiations."

"In the face of this threat, the cost of inaction is unacceptable," she warned.

According to Vitaly Churkin, Russia's ambassador to the United Nations, more than 2,000 people have been killed in the conflict and more than 800,000 have been displaced.