Members of the OSCE examine the scene of a shelling in the town of Donetsk, eastern Ukraine, Wednesday, Aug. 27, 2014. The Obama administration accused Russia on Wednesday of orchestrating a new military campaign in Ukraine, helping rebel forces expand their fight in the country's east and sending tanks, rocket launchers and armored vehicles toward communities elsewhere. (AP Photo/Mstislav Chernov)
Local residents look at a burned car, hit by shelling in the town of Donetsk, eastern Ukraine, Wednesday, Aug. 27, 2014. The Obama administration accused Russia on Wednesday of orchestrating a new military campaign in Ukraine, helping rebel forces expand their fight in the country's east and sending tanks, rocket launchers and armored vehicles toward communities elsewhere. (AP Photo/Mstislav Chernov)
NOVOAZOVSK, Ukraine — Ukraine's president called an emergency meeting of the nation's security council and canceled a foreign trip Thursday, declaring that "Russian forces have entered Ukraine," as concerns grew about the opening of a new front in the conflict.
President Petro Poroshenko summoned the council as the strategic southeastern Ukraine town of Novoazovsk appeared firmly under the control of Russia-backed separatists.
"Today the president's place is in Kiev," Poroshenko 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 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.
Poroshenko canceled a visit to Turkey for the inauguration of newly elected president Recep Tayyip Erdogan and called a snap 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.
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 30 kilometers (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 20 kilometers from the Russian border.
Jim Heintz in Kiev and Laura Mills in Moscow contributed reporting.