Ukrainian President Petro Poroshenko on Monday demanded Russia's Vladimir Putin halt arms supplies to rebels as the leaders of France and Germany tried to revive a peace plan.
After a spike in violence, the four leaders discussed the Ukraine crisis for two hours by telephone in the latest round of talks aimed at stilling a conflict that has killed 10,000 people since April 2014.
The conversation was the first to involve French President Emmanuel Macron since he came to power in May, becoming the main international moderator on the crisis along with Germany's Angela Merkel.
Last week saw some of the costliest clashes in months between Russian-backed rebels and government troops, after an insurgent leader announced plans to form a new "state" to replace Ukraine.
Poroshenko called the last days of July some of the bloodiest in 2017 and urged Russia "to immediately cease aggressive actions and supply of weapons to the occupied territories," Ukraine's presidency said in a statement after the talks.
Kiev and its allies in the West insist the Kremlin has sent troops and arms to back up the rebels, but Moscow continues to deny evidence of its role in the conflict.
In a statement, the Kremlin said Putin "laid out Russia's approach on all the key points of the agreement in great detail."
Separately, the German government said the leaders had agreed on the priority of achieving the withdrawal of troops and heavy weapons from certain zones.
Merkel and Macron "encouraged reciprocal measures, the exchange of prisoners before the end of the year... and the renewal of economic relations," Berlin added.
A February 2015 peace plan hammered out by the leaders of France, Germany, Russia and Ukraine in the Belarusian capital Minsk has failed to stop clashes rumbling on in east Ukraine.
And although the deal has hit a wall, it is still viewed by those involved as the only way of unwinding Ukraine's war.
Last week, rebel chief Alexander Zakharchenko said he wanted to establish a new state called Malorossiya (Little Russia).
While the plan immediately appeared doomed, after it failed to gain traction with the Kremlin, it cast a further cloud over the peace agreement.