Heavy shelling reportedly resumed overnight in Ukraine's separatist-held Donetsk, raising fears about a return to full-scale conflict.

The bombardment was the worst since the September 5 signing of a shaky ceasefire that halted all-out confrontations across most of the conflict zone but hasn't stopped shelling in certain hotspots, witnesses said.

Reuters reports:

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.

According to Agence France-Presse:

The Organisation for Security and Cooperation in Europe (OSCE) voiced concern Saturday after its monitors witnessed unmarked columns of tanks and troop carriers moving through east Ukraine in territory held by pro-Russia separatists.

The sightings of armoured columns came after Ukraine's military said Friday a large convoy of tanks and other heavy weapons entered the country from Russia across a section of border that has fallen under the control of rebel fighters.

Russia denies being involved in the fighting in the east.

The BBC adds that later, OSCE Chairperson-in-Office and Swiss Foreign Minister Didier Burkhalter said he was "very concerned about a resurgence of violence in the eastern regions of Ukraine".

The world "is on the brink of a new Cold War" sparked by Ukraine, former Soviet leader Mikhail Gorbachev said at an event Saturday marking the 25th anniversary of the fall of the Berlin Wall. "Some are even saying that it has already begun."