The U.N. Security Council called an emergency meeting on the Ukraine crisis for Thursday afternoon only hours after Ukrainian President Petro Poroshenko declared that "Russian forces have entered Ukraine."
The meeting, scheduled for 2 p.m. ET, was requested by Lithuania.
Ukraine has charged that at least two convoys of Russian military equipment entered southeastern Ukraine this week to open up a third front in the fighting between Ukrainian armed forces and Russian-backed separatists in eastern regions.
More than 2,000 people have died in fighting in eastern Ukraine, according to a recent U.N. report. Russian-backed rebels have declared two regions as independent republics and the cities of Donetsk and Luhansk have been largely surrounded by Ukrainian forces.
Poroshenko, who met only two days ago with Russian President Vladimir Putin to try to ease tensions in Ukraine, had called for U.N. action in a televised statement to the nation, saying, "The world must provide assessment of sharp aggravation of the situation in Ukraine."
In London, British Prime Minister David Cameron said there is "mounting evidence that Russian troops have made large-scale incursions" into southeastern Ukraine. Such actions are "completely unacceptable and illegal," he added, urging Russia to find a political solution to the crisis or "there will be further consequences."
In Brussels, Brig. Gen. Nico Tak told reporters at NATO headquarters Thursday that the alliance had noted a "significant escalation in both the level and sophistication of Russia's military interference in Ukraine" in the past two weeks.
"Russia is reinforcing and resupplying separatist forces in a blatant attempt to change the momentum of the fighting, which is currently favoring the Ukrainian military," Tak said.
NATO also produced satellite images to provide what it called "additional evidence that Russian combat soldiers, equipped with sophisticated heavy weaponry, are operating inside Ukraine's sovereign territory."
One senior military officer said NATO believes well over 1,000 Russian troops are operating inside Ukraine, Reuters reports, but referred to the Russian actions as "incursions" rather than invasion.
Geoffrey Pyatt, U.S. ambassador to Ukraine, also weighed in on Thursday, writing on Twitter that Russian troops are now directly intervening in Ukraine because of a flagging military effort by rebels.
"Russian supplied tanks, armored vehicles, artillery and multiple rocket launchers have been insufficient to defeat Ukraine' armed forces," Pyatt writes. "So now an increasing number of Russian troops are intervening directly in fighting on Ukrainian territory. "
As charges of a Russian incursion mounted, Andrey Kelin, Russia's representative to the Organization of Security and Cooperation in Europe, which has an international monitoring group in Ukraine, denied the allegations. "We have said that no Russian involvement has been spotted, there are no soldiers or equipment," he said, the Russian news agency ITAR-TASS reports.
"Accusations relating to convoys of armored personnel carriers have been heard during the past week and the week before that," he said. "All of them were proven false back then, and are being proven false again now."
Ukraine also said this week that it had captured 10 Russian paratroopers who had crossed into Ukraine, and showed video of some of the men being interviewed. Putin has denied the allegations of a deliberate incursion, suggesting the soldiers crossed the unmarked border by accident while on training exercises.
After his meeting with Poroshenko in Belarus, Putin said a possible cease-fire plan did not come up. He said a solution to the crisis in east Ukraine is "not our business; it is a domestic matter for Ukraine itself." He said all Russia could do was "support the creation of an environment of trust."
A pro-Russian leader, meanwhile, concedes that as many as 4,000 Russian citizens — including many former high-ranking military officers — are fighting with the rebels, but are doing so strictly voluntarily, many while on leave from the Russian military.
"Many former high-ranking military officers have volunteered to join us. They are fighting with us, considering that to be their duty," Alexander Zakharchenko, prime minister of the self-declared Donetsk People's Republic in Ukraine, told Russian TV, the BBC reports.
"There are also many in the current Russian military that prefer to spend their leave among us, brothers who are fighting for their freedom, rather than on a beach," Zakharchenko said.
Russian markets dived as fears grew that the country was escalating its role in the conflict, a move that could provoke the U.S. and European Union to impose further sanctions on Russian businesses and individuals. Russia's MICEX index dropped nearly 2% on Thursday, and major Russian banks VTB and Sberbank dropped more than 4%.
On Thursday morning, an Associated Press journalist saw rebel checkpoints at the outskirts of the southeastern city of Novoazovsk and was told he could not enter. One of the rebels said there was no fighting in the town.
Novoazovsk, which lies along the road connecting Russia to the Russia-annexed Crimean Peninsula, had come under shelling for three days, with the rebels entering on Wednesday. The southeastern portion of Ukraine along the Azov Sea previously had escaped the fighting engulfing areas to the north.
The new southeastern front raised fears that the separatists are seeking to create a land link between Russia and Crimea. If successful, it could give them or Russia control over the entire Sea of Azov and the gas and mineral riches that energy experts believe it contains. Ukraine already has lost roughly half its coastline, several major ports and significant Black Sea mineral rights in March when Russia annexed Crimea.
In Mariupol, a city of 450,000 about 20 miles to the west, a brigade of Ukrainian forces has arrived at the airport, while deep trenches were dug a day earlier on the city's edge.
In Donetsk, the largest rebel-held city, 11 people were killed by shelling during the night, the city administration said in a statement.
The U.S. government accused Russia of orchestrating a new military campaign in Ukraine that is helping rebel forces expand their fight and sending in tanks, rocket launchers and armored vehicles.
"These incursions indicate a Russian-directed counteroffensive is likely underway in Donetsk and Luhansk," State Department spokeswoman Jen Psaki said. She voiced concern about overnight deliveries of materiel in southeast Ukraine near Novoazovsk and said Russia was being dishonest about its actions, even to its own people.
Russian forces, she said, are being sent 30 miles inside Ukraine, without them or their families knowing where they are going. She cited reports of burials in Russia for those who've died in Ukraine and wounded Russian soldiers being treated in a St. Petersburg hospital.
Associated Press journalists on the border have seen the rebels with a wide range of unmarked military equipment — including tanks, Buk missile launchers and armored personnel carriers — and have run into many Russians among the rebel fighters.
Contributing: Oren Dorell, in McLean, Va.; Associated Press
Follow Doug Stanglin on Twitter @dstanglin