East Ukraine's rebel stronghold Donetsk was pummeled on Sunday by the heaviest shelling in a month, and the OSCE said it spotted an armored column of troops without insignia in rebel territory that Kiev said proved Moscow had sent reinforcements.
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 each side 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. 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 armored 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".