Bystanders watch a fire consuming a school in downtown Donetsk on Wednesday after being hit by a shelling. Several civilians died when their car was completely burned after being hit by shell fragments in central Donetsk, the rebel-held city in eastern Ukraine. (FRANCISCO LEONG / AFP/Getty Images)
Ukrainian President Petro Poroshenko declared Thursday that "Russian forces have entered Ukraine" and called an emergency meeting of the nation's security council to respond to what he said was a "sharp aggravation" in eastern Ukraine, where Ukrainian armed forces are battling separatist rebels.
Poroshenko, who met only two days ago with Russian President Vladimir Putin to try to ease tensions in Ukraine, added that Kiev would shortly call for an emergency meeting of the U.N. Security Council.
"The world must provide assessment of sharp aggravation of the situation in Ukraine," he said.
Poroshenko also canceled a trip to Turkey, saying "today the president's place is in Kiev," according to the president's official web site.
The president's declaration follows Ukrainian charges that columns of Russian military vehicles crossed into Ukraine three days ago to open up a new front in the fighting in southeastern Ukraine, where rebels appear to have firmly taken control of the town of Novoazovsk.
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 denied the allegations, suggesting the soldiers apparently crossed the unmarked border by accident while on training exercises.
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 percent on Thursday, and major Russian banks VTB and Sberbank dropped more than 4 percent.
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 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 arrived at the airport on Wednesday afternoon, 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.
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 which "is not known to have been exported or operated outside of Russia."
The tanks' presence, he added in a blog published Thursday, "strongly supports the contention that Russia is supplying arms to separatist forces."
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 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 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. Ukraine also captured 10 soldiers from a Russian paratrooper division Monday around Amvrosiivka, a town about 12 miles from the Russian border.
Contributing: Associated Press