Heavy fighting erupted around the rebel stronghold of Donetsk in eastern Ukraine on Sunday, putting further pressure on a precarious nine-day-old truce between the government and pro-Russia separatist fighters.
Reporters saw large clouds of thick black smoke billowing over the industrial city amid sustained shelling and gunfire.
Kiev accused the rebels of jeopardising the truce by intensifying attacks on government positions in eastern Ukraine, the scene of five months of deadly combat.
Sunday's fighting appeared to be concentrated near Donetsk airport where the Ukraine military said it had driven back a major assault by insurgent fighters on Friday.
''The terrorist actions are threatening the realisation of the Ukrainian president's peace plan,'' said national security and defence council spokesman Volodymyr Polyovy.
He also criticised the stand of by two rebel leaders who both signed the 12-point truce deal in Minsk on 5 September, but who declared on Sunday they were mere ''observers'' at the talks.
The ceasefire deal has largely calmed a conflict that has cost more than 2,700 lives and set off the worst crisis in East-West relations since the Cold War.
Rebels and government forces have swapped dozens of captives since the accord, and Ukrainian President Petro Poroshenko has pledged to offer limited self-rule to the eastern regions that form the economic backbone of Ukraine.
But the insurgents on Sunday accused Kiev's forces of firing at them.
''From our side, nobody is shooting but they are breaking the rules, everybody in the world knows it,'' said a rebel commander defending a checkpoint near the village of Olenivka south of Donetsk.
The simmering crisis has exposed layers of mistrust between both the West and Moscow and between the largely Russian-speaking populations in the east of Ukraine and the pro-Western leaders in Kiev. Ukrainian Prime Minister Arseniy Yatsenyuk accused Russian President Vladimir Putin on Saturday of keeping Ukraine in a state of war to create a ''frozen conflict'' in Russia's backyard.
''He wants to eliminate Ukraine as an independent country,'' Yatsenyuk said.
The West has been acting to isolate Putin, who is seen as less predictable and more aggressive than at any point since his domination of Russia began 15 years ago, and has pledged greater support for the government in Kiev.
Poroshenko heads to Washington this week to meet President Barack Obama, seeking to secure a ''special status'' alliance with the United States as he steers Ukraine further out of Russia's orbit.
Obama has rejected direct military involvement but instead unveiled increasingly painful economic sanctions on Moscow that, along with similar EU measures, effectively lock Russia out of Western capital markets and hamstring its crucial oil industry.
Russian Foreign Minister Sergei Lavrov accused Washington of ''trying to use the crisis in Ukraine to break economic ties between the EU and Russia and force Europe to buy US gas at much higher prices''.
The punitive measures and an accompanying East-West trade war have left Russia's economy facing an increasingly gloomy future and a possible recession this year, but have seemingly failed to dent Putin's popularity in the country.