Ukraine crisis: 'Rebel reinforcements' raise fears
The US and the European Union have expressed concern at reports of a build-up of separatist reinforcements in eastern Ukraine.
Shelling around the city of Donetsk on Sunday followed an intense overnight artillery bombardment, shaking an already fragile ceasefire.
Monitors and journalists reported unmarked convoys of heavy weapons, tanks and fighters close to the city.
Russia denies claims that it is supplying the separatist rebels.
The EU's head of foreign policy, Federica Mogherini, said the latest reports were "worrying".
"The most recent reports by the OSCE [Organisation for Security and Co-operation in Europe] Special Monitoring Mission in Ukraine about convoys in separatist held areas with substantial amounts of heavy weapons, tanks and troops without insignia moving westwards represent a very worrying development," she said.
The White House warned that any efforts by pro-Russian rebels to seize more territory would be a "blatant violation" of the ceasefire agreement signed in Minsk in September.
"We are very concerned by intensified fighting in eastern Ukraine, as well as numerous reports... that Russian backed and supplied separatists are moving large convoys of heavy weapons and tanks to the front lines of the conflict," said US National Security Council spokeswoman Bernadette Meehan.
"We continue to call on all sides to strictly adhere to the ceasefire. Any attempt by separatist forces to seize additional territory in eastern Ukraine would be a blatant violation of the Minsk agreements."
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.
Renewed shelling around Donetsk started four days ago, just after elections in rebel areas, but the barrage that took place overnight into Sunday was said to be very heavy.
A lull in the fighting followed but shelling later resumed. No casualties have been reported so far.
The 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 and Swiss Foreign Minister Didier Burkhalter said he was "very concerned" about the resurgence of violence.
Fears of a return to full-scale conflict were raised after the separatists held elections a week ago 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.