The Organization for Security and Cooperation in Europe says both sides are building up tanks,firing lines,and moving weapons in violation of a February agreement.
The Russian-backed rebels claim Ukrainian forces are the aggressors and planning more attacks-a charge Kyiv throws back at them.
Rebel Colonel Alexander Kurinkov with the Oplot fifth mobile infantry artillery brigade denies any build-up.
“If they think it's around Donetsk then let them go and see for themselves.But they're not there.”
The colonel also repeats the Kremlin line-denying any Russian troops or weapons are involved except volunteers-despite mounting evidence otherwise.
But Kurinkov admits they plan to take more territory by negotiation or by military force.
“How can we stop at these borders that we have now when the fighters that fight for their-let us say-new country, their families and their roots, are still in the occupied territories?”
Ukrainian forces say they fought back a planned rebel assault near the town of Marinka while rebels claim Kyiv provoked the fighting.
Clashes have delayed delivery of humanitarian aid produced in Ukrainian-controlled territory,said Olga Tseselskaya of the aid group Pomozhem.
The Donbas Arena, home of the Shaktar Donetsk football team,distributes the aid.But while supplies are running low,Tseselskaya said the need remains high.
“The first reason is the lack of financial help,for example,payments of pensions and social payments to the most needy civilians.And,second is high prices and lack of medicines and food products.”
The football team last year moved to safety in Kyiy.
The stadium has been hit by shelling.
But Tseselskaya said the volunteer workers will keep helping locals as long as they can.
“At this very moment the situation is so unstable that nobody can say how long the situation can last or how long our humanitarian aid will be needed in the form it exists right now.”
If fighting escalates, the help could be delayed further.