Russian troops fighting on the side of separatists in Ukraine are advancing in the southeast region of the country, Ukraine President Petro Poroshenko said on Thursday.

"I have made a decision to cancel my working visit to the Republic of Turkey due to sharp aggravation of the situation in Donetsk region, particularly in Amvrosiivka and Starobeshevo, as Russian troops were brought into Ukraine," Poroshenko said in a statement.

"Russia, Ukraine now at war," tweeted geopolitical expert Ian Bremmer of Eurasia Group, adding that "Ukraine economy will collapse if direct fighting [with] Russia persists; support from the West isn't there. Poroshenko now needs a ceasefire."

The United Nations Security Council is holding an emergency meeting Thursday afternoon in New York on the escalation in Ukraine at the request of Lithaunia, a NATO state which shares a border with Russia.

"There's an invasion of Russian troops into the east of Ukraine," Dainus Baublys, the minister counselor for Lithuania's UN mission, told Business Insider. "Up to 1,000 Russian troops invaded yesterday ... they continue pounding Ukrainian forces across the border with their artillery and they are deploying more artillery pieces along the border."

The new front opened in Amvrosiivka and Starobeshevo opens a pathway to Crimea, which Russia annexed with special forces troops in March.



The fear is that Russia is attempting to create a land link between Russia and the strategic peninsula.

Russian troops are leading a separatist counteroffensive in the east, bringing in tanks and using artillery from inside Ukrainian territory.

A Russian-backed rebel leader said that at least 3,000 to 4,000 Russian troops were fighting inside Ukraine.

"Current servicemen are also fighting in our ranks, as they came to us to struggle for our freedom instead of their vacations," prime minister of the self-proclaimed Donetsk People’s Republic (DPR) Alexander Zakharchenko told Russian media. "This is characteristic only for Russians."

Reports from the ground describe an escalating ground war.

"This is what happened: They crossed the border, took up positions, and started shooting," Sgt. Aleksei Panko said after telling The New York Times on Wednesday that about 60 armored vehicles crossed the border near the town of Novoazovsk. "This is now a war with Russia."

Novoazovsk, south of Amvrosiivka and Starobeshevo, is on the highway linking Russia to the Ukrainian port of Mariupol and onto Crimea. Ukraine's military said Russian troops had taken control of Novoazovsk and several other settlements in the south of Donetsk region.

"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," Ukraine's national security and defense council said in a statement.

A look at the fighting in Ukraine's southeast, where until this week Ukrainian forces were re-taking territory very quickly over the summer. REUTERS

U.S. Ambassador to Ukraine Geoffrey Pyatt also said Russian soldiers were directly involved in the new offensive. And Daniel Baer, the U.S. Ambassador to the Organization for Security and Cooperation in Europe, called the latest escalation a "man-made tragedy, directed by the Kremlin."

"Russian supplied tanks, armored vehicles, artillery and multiple rocket launchers have been insufficient to defeat Ukraine's armed forces, so now an increasing number of Russian troops are intervening directly in the fighting on Ukrainian territory," Pyatt wrote on Twitter.

"Russia has also sent its newest air defense systems including the SA-22 into eastern Ukraine and is now directly involved in the fighting."

Ukrainian Prime Minister Arseniy Yatsenyuk has called for an emergency U.N. Security Council meeting to deal with the issue of Russian troops in Ukraine, although Russia sits on the council and could veto any proposal that arose.

"The Russian military’s use of artillery from locations within Ukraine is of special concern to Western military officials, who say Russian artillery has already been used to shell Ukrainian forces near Luhansk," Andrew Kramer and Michael Gordon of The Times reported Wednesday. "And along with the antiaircraft systems operated by separatists or Russian forces inside Ukraine, the artillery has the potential to alter the balance of power in the struggle for control of eastern Ukraine."

State Department spokeswoman Jen Psaki said Wednesday that Russia’s "incursions" in Ukraine indicated "a Russian-directed counter-offensive is likely underway," which she said was of "deep concern" to the U.S.

She accused Russia of "sending its young men into Ukraine, but not telling them or their parents where they are going or what they are doing."

"These are not steps you take when you are operating in a transparent manner," Psaki said. "We are also concerned about the Russian government’s unwillingness to tell the truth even as its soldiers are found 30 miles inside Ukraine.”

Russian servicemen drive armored vehicles on the outskirts of the city of Belgorod near the Russian-Ukrainian border on April 25. Stringer ./REUTERS

On Tuesday, dozens of heavily armed strangers with Russian accents and military gear without insignias appeared in an eastern Ukrainian village east of the rebel stronghold of Donetsk and set up a roadblock.

"The people at the new checkpoint, they were polite military men wearing green. Definitely not Ukrainian. They're definitely not from around here," one man told Reuters.

The "green men" are a reminder of the Russian special forces that were used to annex the Ukrainian peninsula of Crimea.

The Times noted that Russian President Vladimir Putin, who was in Belarus this week for talks with Ukrainian and European leaders, may be "calculating that Moscow could intervene in eastern Ukraine with conventional Russian forces without risking further Western economic sanctions."

Yatsenyuk suggested the response from the West needed to be tougher, saying that U.S. and European Union sanctions leveled on Moscow had not been "helpful" in keeping Putin from continuing to escalate the situation. He said measures like freezing Russian assets and banning Russian bank transactions should be considered as new punitive measures.



"Vladimir Putin has purposely started a war in Europe. It is impossible to hide from the fact," Yatsenyuk said.

Russian President Vladimir Putin, left, shakes hands with his Ukrainian counterpart Petro Poroshenko as Kazakh President Nursultan Nazarbayev stands nearby, in Minsk on Tuesday. REUTERS/Sergei Bondarenko/Kazakh Presidential Office/Pool

Here's a look by province, and what else Russia would have to annex to create the land bridge:

REUTERS

And here's the fighting as of today, according to Ukraine's military:







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Armin Rosen contributed to this report