Shelling has resumed in the eastern Ukrainian city of Donetsk, just hours after an intense artillery bombardment throughout the night.
An eyewitness said the overnight shelling was the worst in more than a month, lasting almost eight hours.
It appeared to be coming from both rebel and government positions.
The fighting comes a day after monitors saw large numbers of military vehicles in rebel areas. Ukraine says it thinks the rebels get equipment from Russia.
Moscow denies this, although in the past it has said that Russian soldiers on leave were fighting among the rebels.
More than 4,000 people have died since fighting erupted in April after pro-Russian separatists seized control in the regions of Donetsk and Luhansk.
A fragile ceasefire has been in place since 5 September, although hundreds of people have been killed since then.
'Resurgence of violence'
The shelling in Donetsk started four days ago, just after elections in rebel areas, but the round which took place overnight into Sunday was extremely heavy.
A lull in the fighting followed, but shelling later resumed that was less intense. No casualties have been reported so far.
The Organisation for Security and Cooperation in Europe (OSCE) said its observers had seen a column of vehicles and heavy guns in rebel-held areas around Donetsk.
"More than 40 trucks and tankers" were seen on a road east of Makiivka, the monitors said, adding that 19 were large vehicles towing howitzer artillery systems and "containing personnel with dark green uniforms without insignia".
Nine tanks were also seen south-west of the centre of Donetsk.
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".
Fears of a return to full-scale conflict were raised after the separatists held elections on Sunday in defiance of Ukraine's government and Western countries.
Russia has recognised the vote which led to separatist leader Alexander Zakharchenko, 38, being sworn in as head of the self-declared Donetsk People's Republic.
Igor Plotnitsky, a 50-year-old ex-Soviet army officer, was declared head of the self-proclaimed Luhansk People's Republic, further to the east.
President Petro Poroshenko has accused the rebels of tearing up the peace deal and said that a law granting the rebel-held regions partial autonomy would be scrapped. He has ordered reinforcements to key cities in case of a rebel offensive.
But the separatists hit back on Wednesday, arguing that it was the scrapping of the special status deal that broke the peace agreement.