French President Francois Hollande and US President Barack Obama are pressing for a meeting between rebels and the government in Ukraine "as quickly as possible" to negotiate a bilateral ceasefire, Hollande's office said.
Hollande's office said in a statement that the two leaders spoke by phone on Monday and called on Russian President Vladimir Putin to press separatists to accept dialogue with Ukrainian authorities.
Ukrainian President Petro Poroshenko promised to start talks on a new ceasefire last week. A contact group for the Organisation for Security and Cooperation in Europe met in Kiev on Sunday to discuss the situation in Donetsk, but no representatives from the rebels attended and no breakthroughs were announced.
Obama and Hollande called for a meeting "as quickly as possible with separatist elements, with the view to reaching a bilateral ceasefire", the statement said. "A durable solution to the crisis in Ukraine can only be a political one."
The White House said Obama told Hollande that in the absence of meaningful efforts by pro- Russian separatists to pursue peace talks or stick to a ceasefire, the US respects Ukraine's responsibility "to maintain public order in the country and to protect the population".
There are fears that the major cities of Donetsk and Luhansk could be the next focus of serious fighting. Three bridges on roads leading to Donetsk were blown up on Monday - possibly to hinder military movements, though the rebels claim it was the work of pro-Kiev saboteurs.
Battles between Ukrainian forces and the Russia-backed separatists have left over 400 people dead and thousands homeless since the uprising began in April.
Nato is meanwhile drawing up plans to ensure its members can respond more quickly to crises in the aftermath of Russia's "aggression" in Ukraine, the alliance's chief said on Monday.
Secretary General Anders Fogh Rasmussen said the transatlantic alliance would review a proposed readiness "action plan" at an upcoming summit in September in Britain and also predicted European members were poised to reverse a long decline in military spending dating back to the cold war.
"Obviously Russia's aggression against Ukraine will put a lot of emphasis on the need for a strong, collective defence," Rasmussen said during a visit to Washington. "That's why at the summit I hope we will adopt a readiness action plan which will improve our ability to respond swiftly," he told an audience at a think tank, the Atlantic Council.
"Nato is an insurance policy against instability. All members must pay their premiums. And that premium has just gone up," he said.
Agence France-Presse, Associated Press