Ukrainian President Petro Poroshenko has warned of a "colossal threat" of renewed fighting with pro-Russian rebels after a surge in violence that left at least 26 people dead.
Fresh clashes erupted near the rebel stronghold of Donetsk on Wednesday, in one of the worst outbreaks of violence since a European-brokered truce was agreed in February.
The upsurge in fighting revived fears in Kiev that the separatists may be gearing up for a new push into government-controlled territory.
In an annual address to parliament yesterday, Ukrainian President Petro Poroshenko warned of the "permanent danger" of Russia unleashing "a full-blown war" against Ukraine.
"There remains a colossal threat of resumption of large-scale fighting on the part of Russian terrorist groups," Poroshenko said, referring to the insurgents.
The Ukrainian president claimed that more than 9,000 Russian soldiers were now in east Ukraine to support the rebellion and that Moscow kept supplying separatist forces with new weapons.
The United States, the EU and Germany expressed alarm over the new spiral of violence, with European Commission spokeswoman Maja Kocijancic calling the clashes "the most serious violation" yet of the February ceasefire.
"The truce is fragile, the violations can lead to us again falling back into a state of military escalation," said German Foreign Minister Frank-Walter Steinmeier.
Charles Kupchan, the US National Security Council's senior director for European affairs, told reporters "we are particularly concerned about the uptick in fighting that we have seen over the last 24 to 48 hours in Ukraine."
The UN Security Council will hold an emergency meeting today to discuss the escalation, while NATO chief Jens Stoltenberg sounded the alarm over Russia's increasingly aggressive behaviour.
"What we see is increased unpredictability, increased insecurity, increased nervousness," the former Norwegian prime minister said.
The Kremlin accused Kiev of seeking to torpedo the truce ahead of an EU summit in June which will discuss whether to extend sanctions against Russia that are set to expire in July.
"No doubt these provocations are being organised by the Ukrainian army," Russian President Vladimir Putin's spokesman Dmitry Peskov told reporters.
"We are seriously worried by this indeed."
Russian Foreign Minister Sergei Lavrov said the peace deal faced a "constant threat" from the Ukrainian authorities.
