Ukrainian President Petro Poroshenko had earlier said he hoped that a leaders' meeting of the four countries could take place Thursday in Kazakhstan with the goal of signing a peace document with Russia.
The foreign ministers of Ukraine, Russia, Germany and France ended a meeting Monday without being able to set a date for a hoped-for peace summit on ending the Ukrainian conflict.
However, the four ministers were only able to affirm their "strong commitment to a swift and comprehensive peaceful solution of the current crisis, including a sustainable political process".
"Ministers reconfirmed that the basis for conflict resolution are the Minsk agreements that have to be implemented by all sides in their entirety," they said in a joint statement after about four hours of talks in Berlin.
German Chancellor Angela Merkel had at the weekend told Russian President Vladimir Putin and Poroshenko that a peace summit was pointless as long as a truce was not respected, and France has expressed similar reservations.
"The situation in Ukraine remains extremely tense," German Foreign Minister Frank-Walter Steinmeier said before starting the talks with his Russian counterpart Sergei Lavrov, Ukraine's Pavlo Klimkin and Laurent Fabius of France.
"Because there is still no permanent ceasefire, people are dying in eastern Ukraine, and the humanitarian situation ... remains very worrying," Steinmeier said, stressing that "we still have no sustainable path out of the crisis".
After the talks he said that the top diplomats had "a very long, very open exchange, with controversies" and that "the differences of opinion that still exist showed how difficult it is to achieve progress" toward a political solution and a summit.
War has killed over 4,700
All sides had called for new meetings of the Ukraine contact group, and for continued dialogue between their four capitals, with the hope of holding another ministerial meeting next week, Steinmeier said.
The four ministers "reiterated the necessity to fully respect" the ceasefire agreement from September for the east of the former Soviet satellite state.
Mediators in the conflict have demanded Ukraine and Russia implement fully the so-called Minsk peace accord which pro-Russian separatist guerrillas and Ukrainian forces have violated repeatedly in a war that has already killed more than 4,700 people.
Both sides in Ukraine accuse each other of being at fault for the impasse.
The rebel stronghold of Donetsk -- a once bustling city of nearly a million that now stands half vacant and suffers chronic power and water shortages -- has been shaken by heavy artillery fire in the past week.
The Ukrainian government accuses the insurgents of escalating attacks to scuttle implementation of the September peace settlement. This would include forcing Russia to return control to Ukraine of a long section of border, inflicting a severe blow to the Russian-backed rebels' logistics.
Ukraine's armed forces said the militias attacked federal positions on 63 occasions since Sunday morning.
The war erupted last year shortly after crowds in Kiev overturned the country's Moscow-backed president. Russia then annexed the southern province of Crimea and insurgents in the east began their uprising.
Many Ukrainians want Poroshenko to restore control over the whole country, but the Ukrainian army is in tatters after years of underfunding and corruption.
The war is also draining what little remains of Ukraine's financial resources at the same time as the country tries to embark on tough internationally-backed economic reforms.
Putin -- his personal approval soaring but his country's economy shrinking for the first time since 2009 -- needs to scale down support for the pro-Russian insurgents in order to persuade the United States and EU governments to ease off on sanctions.
Some Europeans have expressed alarm over the potential security risks posed by the nuclear superpower's economic troubles.
"I think the sanctions must stop now," French President Francois Hollande said last week. "They must be lifted if there is progress."
However, Russia's Lavrov insisted Monday that Moscow's Ukraine policy would not depend on Western pressure.
"We were not the ones who got the sanctions ball rolling," Lavrov said. "This is their problem."
By Daniel Aronssohn (AFP)