East Ukraine's rebel stronghold Donetsk was pummelled on Sunday by the heaviest shelling in a month, and the OSCE said it spotted an armoured column of troops without insignia in rebel territory that Kyiv said proved Moscow had sent reinforcements.
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A two-month-old ceasefire to end a war that has killed 4,000 people has appeared shakier than ever in the past few days, with both sides accusing the other of having violated the terms of the peace plan.
Reuters journalists inside Donetsk, who have been there throughout the fighting, said the shelling sounded more intense than at any time since early October, a period when a playground was struck killing at least 10 people. Sunday's strikes appeared to come from territory held by both government and rebel forces.
Ukraine's military said its standoff with the Russian-backed separatists in the east had intensified in the past week, which saw the rebels swear in new leaders after elections the government says violated the terms of the truce pact.
Ukraine has accused Russia of sending a column of 32 tanks and truckloads of troops into the country's east to support the pro-Russian rebels in recent days. Moscow has long denied its troops operate in east Ukraine, although many have died there.
The Organization for Security and Cooperation in Europe (OSCE), which includes Russia and Ukraine as well as the United States and NATO countries, operates in East Ukraine with the blessing of all sides and is widely seen as neutral.
Its statement that it spotted an unidentified armoured column in rebel territory helps support Kiev's position that Moscow has been sending in reinforcements to protect separatist enclaves the Kremlin now refers to as "New Russia."
Ukraine certain new troops are Russian
In one 40-vehicle convoy, "19 were large trucks — Kamaz type, covered, and without markings or number plates — each towing a 122-mm howitzer and containing personnel in dark green uniforms without insignia," the watchdog said in statement.
Ukraine said it had no doubt the new troops were Russians.
"Although the OSCE did not specify to whom the equipment and soldiers belonged, the Ukrainian military has no doubt of their identity," said military spokesman Andriy Lysenko.
"The past week was characterized by an increase in the intensity of shelling and the transfer of additional force: ammunition, equipment and personnel, to terrorist groups," Lysenko said.
Reuters reporters in rebel-held Donetsk said intense shelling by heavy artillery continued throughout the night and into the early hours, and then picked up again later on Sunday morning. The shelling could be heard in the centre of the city, which had a pre-conflict population of more than 1 million.
Large clouds of black smoke could be seen over the ruins of the airport, which is still under government control but which the separatists are seeking to seize.
There were no immediate reports of new civilian casualties. Lysenko said three Ukrainian soldiers had been killed in the past 24 hours and a further 13 injured.
OSCE Chairman Didier Burkhalter said on Saturday he was "very concerned about a resurgence of violence in the eastern regions of Ukraine and about activities leading to more fragility instead of further stabilization of the situation."
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He urged both sides to stick to the agreements reached in a 12-point ceasefire deal on Sept. 5.
Lysenko said Ukraine's military believes Russia could stir up tension to provide grounds to "send in so-called Russian peace-keeping units".
The United States and European Union have imposed economic sanctions on Moscow over Ukraine since March, when Russia seized Ukraine's Crimea peninsula. Moscow has since backed separatists who rose up in east Ukraine, while denying the presence of its own troops.
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The sanctions have hurt Russia's economy, already facing a fall in the price for its oil exports, and have helped drive a crash in the value of the Russian rouble.
U.S. Secretary of State John Kerry said on Saturday the United States and Russia had agreed to exchange information about the situation on the Russia-Ukraine border due to some "some disagreements about some of the facts on the ground".
Although Russia blames the crisis on Kyiv and the West, NATO has said it has overwhelming evidence that Russia has aided the rebels militarily in the conflict.
On Saturday, investigative journalists published a report on the downing of a Malaysian airplane over rebel territory in July in which 298 people died. The Bellingcat report said there was "strong evidence indicating that the Russian military provided separatists in eastern Ukraine with the Buk missile" believed to have shot down the plane.