Last Updated Aug 28, 2014 9:00 AM EDT
NOVOAZOVSK, Ukraine -- "Russian forces have entered Ukraine," the country's president declared Thursday, cancelling a foreign trip and calling an emergency meeting of his security council. Reports from analysts to separatist rebels to NATO and U.S. officials backed up that assessment.
President Petro Poroshenko summoned the council as the strategic southeastern town of Novoazovsk appeared firmly under the control of separatists and their Russian backers, a new front in the war in eastern Ukraine between the separatists and Poroshenko's government in Kiev.
"Today the president's place is in Kiev," Poroshenko said.
Russian stock 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 percent on Thursday, and major Russian state banks VTB and Sberbank dropped more than 4 percent.
The leader of the insurgency, Alexander Zakharchenko, said in an interview on Russian state television that 3,000 to 4,000 Russians have fought on the separatist side since the armed conflict began in April.
The Reuters news agency reports a member of Russian President Vladimir Putin's advisory council on human rights said Thursday she believed Russia was carrying out an invasion of Ukraine.
"When masses of people, under commanders' orders, on tanks, APCs and with the use of heavy weapons, (are) on the territory of another country, cross the border, I consider this an invasion," Ella Polyakova told Reuters.
A NATO military officer in Brussels told The Associated Press that "we assess there are over 1,000 Russian troops operating inside Ukraine" now. He said NATO estimates that another 20,000 Russian troops are close by, right over the Russian border. He spoke Thursday on condition of anonymity because the material was confidential.
The U.S. government accused Russia of orchestrating a new military campaign in Ukraine, 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," U.S. State Department spokeswoman Jen Psaki said Wednesday. 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 (48 kilometers) inside Ukraine, without them or their families knowing where they are going. She cited reports of burials in Russia for those who have died in Ukraine and wounded Russian soldiers being treated in a St. Petersburg hospital.
On Thursday morning, an Associated Press journalist saw rebel checkpoints at the outskirts 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 it on Wednesday. This area had previously escaped the fighting that has engulfed areas to the north, and the only way rebels could have reached the southeast, which was under government control, was coming through Russia.
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 lost roughly half its coastline, several major ports and significant Black Sea mineral rights in March when Russia annexed Crimea.
Poroshenko canceled a visit to Turkey for the inauguration of newly elected President Recep Tayyip Erdogan and called an emergency session of Ukraine's security council.
"I have decided to cancel my visit to Turkey because of the sharp escalation of the situation in the Donetsk region ... as Russian forces have entered Ukraine," he said.
In Mariupol, a city of 450,000 about 30 kilometers (20 miles) to the west of Novoazovsk, a brigade of Ukrainian forces arrived at the airport on Wednesday, while deep trenches were dug a day earlier on the city's edge.
National Guard spokesman Ruslan Muzychuk told The Associated Press in Mariupol that the government has evidence that large amounts of weapons have been brought into Novoazovsk from across the Russian border.
"At the moment, there is a large contingent of military machinery as well as mobile saboteur groups firing artillery at Ukrainian forces" in Novoazovsk, Muzychuk said, adding that National Guard reinforcements were taking up positions in Mariupol.
"The positions are being strengthened," the spokesman said. "The road from Novoazovsk to Mariupol is under the control of Ukrainian troops."
Joseph Dempsey, an analyst at the International Institute for Strategic Studies in London, said recent images of a military convoy in eastern Ukraine showed the presence of a variant of the T-72 tank that "is not known to have been exported or operated outside of Russia."
The tanks' presence, he added in note Thursday, "strongly supports the contention that Russia is supplying arms to separatist forces."
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. Ukraine also captured 10 soldiers from a Russian paratrooper division Monday around Amvrosiivka, a town 20 kilometers (12 miles) from the Russian border.
In Donetsk, the largest rebel-held city, 11 people were killed by shelling overnight, the city said Thursday.