More than 20 people have been reported killed in separate attacks in eastern Ukraine, as leaders of Ukraine, Russia, Germany, and France met for peace talks in Minsk, Belarus.
Shells struck a bus station in the rebel stronghold of Donetsk, Al Jazeera's Charles Stratford reported from the region, as fighting continued to rage in the country's east in the run-up to Wednesday's talks.
Rebel officials said that five people were killed and nine wounded at the scene of the attack, where an Associated Press news agency reporter saw one body.
Donetsk city officials said in a statement that three people had been killed in the shelling overnight.
Ukrainian President Petro Poroshenko said on Wednesday he was ready to introduce martial law throughout the country if the conflict with pro-Russian separatists in the east escalated further.
"I will not hesitate with this decision, if the actions of the aggressor lead to further escalation," he told a cabinet meeting before the peace talks in Minsk.
'Most important meeting'
Poroshenko said the Minsk summit was "the most important meeting" of his presidency.
"The summit's outcome will decide whether we can stop the aggression through diplomatic channels or if we switch to a completely different mode," he said.
He posted a statement on his website saying that he had made an impromptu visit to war-torn eastern Ukraine early on Wednesday.
Poroshenko stopped in the city of Kramatorsk, about 50km from the nearest front line, where 16 people were killed and 48 wounded, according to his government, in a rocket strike on Tuesday.
Officials in Kiev also said on Wednesday that 19 troops had been killed and 78 wounded in a day of fighting in Debaltseve, a hotly contested transport hub in the region.
ANALYSIS: Russia stands firm as Minsk talks begin
Poroshenko met with Russian President Vladimir Putin, French President Francois Hollande and German Chancellor Angela Merkel for the planned peace talks, hosted by Belarus President AlexanderLukashenko.
France and Germany, who are brokering the talks, have called on the rival parties to refrain from hostilities that could derail the four-way summit.
Obama-Putin phone call
Al Jazeera's Rory Challands, reporting from Minsk, said there was a "degree of pessimism" among the Germans, because of the "huge amount of fingerpointing and acrimony" between Russia and Ukraine.
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On Tuesday, US President Barack Obama urged his Russian counterpart Vladimir Putin in a phone call to agree to a peace deal with the Ukrainian government.
"If Russia continues its aggressive actions in Ukraine, including by sending troops, weapons, and financing to support the separatists, the costs for Russia will rise," the White House said in a statement.
The Kremlin, however, emphasised in its statement about the call that the Ukraine crisis was an "internal" conflict.
Ukraine and the West accuse Russia of supplying and training the heavily armed separatists, but Russia denies the claims.
On Wednesday, the top US military commander in Europe Lieutenant-General Ben Hodges said the US military will train three battalions of Ukrainian soldiers, who are battling the Russian-backed separatist rebels.
Source: Al Jazeera and agencies