Germany's foreign minister has expressed "grave concern" over the escalation of violence in eastern Ukraine. A flare-up in fighting is threatening to dismantle an already fragile cease-fire brokered four months ago.
German Foreign Minister Frank-Walter Steinmeier warned Thursday that there had been "serious violations" of the truce deal, which could lead eastern Ukraine closer to full-scale war.
"I've always said the situation is not so stable that it couldn't veer out of control any day," Steinmeier told reporters in Berlin, following a meeting with his Ukrainian counterpart Pavlo Klimkin.
At least 26 people were killed after fresh clashes erupted Wednesday between government forces and pro-Russian separatists outside the rebel stronghold of Donetsk.
Steinmeier called on both sides to return to the basis of the February cease-fire brokered by France and Germany in Minsk, which requires heavy weapons to be pulled back from the front line to create a buffer zone.
"The truce is fragile, the violations can lead to us again falling back into a state of military escalation… We must prevent that," Steinmeier said.
According to Kyiv, Wednesday's fighting - the deadliest seen in months - was set off by a large-scale rebel assault on the government-held town of Marinka, about 30 kilometers (19 miles) west of Donetsk. The rebels, however, blamed Ukrainian forces for starting the attack.
Military sources said a relative calm had returned to Marinka by Thursday morning.
Breaking the truce
The February truce has been frequently violated by low-level skirmishes in eastern Ukraine, but the latest clashes have included the use of heavy weapons on both sides.
Observers from the Organization for Security and Cooperation in Europe (OSCE), responsible for monitoring the implementation of the ceasefire, reported seeing at least a dozen tanks moving westward from Donetsk in the lead-up to the battle at Marinka.
OSCE mission spokesman Michael Bociurkiw said some heavy weapons, which had previously been pulled back by both sides, were now missing from the where they had been stored.
"This suggests non-compliance" with the cease-fire accord, Bociurkiw said.
Following the latest surge in violence, Ukrainian President Petro Poroshenko warned Thursday of a "colossal threat" of large-scale military operations "from the side of the Russian-terrorist groups" in eastern Ukraine.
In his annual address to parliament, he added that there were estimated to be more than 9,000 Russian troops on Ukrainian territory and that Ukraine's army needs to be ready for a "full-scale invasion" along the country's entire border with Russia.
The UN Security Council is expected to hold an emergency meeting Friday to discuss the conflict's recent escalation. More than 6,400 people have been killed in the fighting in eastern Ukraine since April 2014.