Fresh reports of violence and convoys of heavy weaponry on the move in eastern Ukraine on Monday increased doubts about the viability of a shaky cease-fire reached between Kyiv and Russia-backed separatists in September.
The Donetsk regional administration reported that three civilians and a Ukrainian serviceman were killed and seven other people wounded Monday when separatist forces shelled a northern suburb of the city of Donetsk.
The rebel-held city was heavily bombarded Sunday, with Reuters journalists reporting from inside Donetsk that the shelling appeared more intense than at any time since early October. They said the fire was coming from an area of intense combat, and reported large plumes of smoke over the ruins of the city's airport.
Each side has repeatedly accused the other of shelling civilian areas.
Western journalists reported Monday seeing an armored column including at least six tanks moving toward Donetsk city.
Organization for Security and Cooperation in Europe (OSCE) monitors in Ukraine expressed concern after seeing convoys of heavy weapons and tanks in separatist-controlled eastern Ukraine. On Friday, Kyiv accused Moscow of sending more than 30 tanks and truckloads of troops to support the pro-Russian rebellion.
Germany's Foreign Minister Frank-Walter Steimeier said during a visit to Kazakhstan on Monday that the situation in eastern Ukraine has again become serious and that everyone involved in the conflict, including Russia, should resume the fulfillment of the cease-fire agreed to in the Belarussian capital of Minsk in September.
The White House voiced "grave concern" Sunday over the latest upsurge in fighting and movement of heavy weaponry.
And U.S. National Security Council spokeswoman Bernadette Meehan warned that any move by pro-Russian rebels to seize more territory in Ukraine's embattled east would be seen as a "blatant violation" of the cease-fire deal.
Moscow has repeatedly denied any official military involvement in Ukraine, and has described Russian troops fighting alongside rebels as volunteers.
Meanwhile, Turkish Foreign Minister Mevlut Cavusoglu said Monday that his country will never recognized what he called Russia's "illegal annexation" of Crimea from Ukraine in March. He made his comments after meeting with Ukrainian Foreign Minister Pavlo Klimkin in Kyiv.