The four leaders also agreed in a telephone call that hostages held in eastern Ukraine should be freed without delay, Chancellor Angela Merkel's spokesman, Steffen Seibert said.
Observers from the Organization for Security and Cooperation in Europe have been held in Ukraine for weeks.
Seibert didn't say how a cease-fire oversight mechanism might work but said talks between Russia, Ukraine and the OSCE should continue.
The four leaders spoke after the upper house of the Russian parliament canceled a resolution allowing the use of military in Ukraine at Russian President Vladimir Putin's request. The move was intended to show Moscow's eagerness to de-escalate tensions and avoid a new round of sanctions at Friday's European Union summit.
Putin has said the cancellation is intended to help support the peace process in Ukraine, which began Friday with a weeklong cease-fire.
However, in recent days the cease-fire has appeared to be in jeopardy as violence has continued in parts of eastern Ukraine. Local residents said fighting was raging as recently as Wednesday morning around the city of Slovyansk where at least one woman died when a mortar bomb tore through the roof of her house. AP reporters who visited Slovyansk saw fresh damage from the fighting and witnesses said that rebels fired on government forces' positions just outside the city, drawing retaliatory fire that damaged some residential buildings.
On Tuesday Poroshenko warned that he may end the truce ahead of time after rebels, who had pledged to respect the cease-fire, used a shoulder-fired missile to down a Ukrainian military helicopter, killing nine servicemen.
Earlier Wednesday, German Chancellor Angela Merkel said Putin's move to rescind the permission for using military force in Ukraine was an "important psychological point," but that progress toward a solution remains slow and EU is still prepared to increase sanctions.
Merkel said the EU will do everything possible to help find a diplomatic resolution, but added that "if nothing else helps, sanctions could return to the daily agenda, and this time at the third level."
Two previous rounds of U.S. and EU sanctions imposed asset freezes and travel bans on members of Putin's inner circle over Russia's annexation of Crimea. The next round, which would impose penalties for entire sectors of the Russian economy, could be far more crippling.
U.S. administration officials said, however, that the new sanctions could be delayed as European leaders appeared less enthusiastic about them amid positive signals from Putin.
The Russian parliament vote came as NATO foreign ministers gathered in Brussels, warning that Russia could face further punishment and considering ways to bolster Ukraine's military.
Speaking after the meeting, NATO's secretary general, Anders Fogh Rasmussen, urged Russia to "take genuine and effective measures to stop destabilizing Ukraine ... create conditions for the peace plan to be implemented ... end its support for armed separatist groups, and ... stop the flow of weapons and fighters across its borders."
"This is a real opportunity to de-escalate the crisis caused by Russia's aggression, and Russia must step back in line with its international obligations," Rasmussen said.
He added that the meeting approved moves to help build Ukraine's military capacities, including by creating trust funds to support defense capacity building in critical areas such as logistics, command and control, and cyber defense.
On Tuesday, Putin urged Ukraine to extend the truce and sit down for talks with the rebels. He argued that the Ukrainian demand that the insurgents lay down their weapons within a week was unrealistic, explaining that they would be reluctant to disarm for fear of government reprisals.
The Russian leader also called on Ukraine to adopt constitutional amendments and other legal changes that would protect the rights of Russian-speakers in the east.
Poroshenko promised Wednesday that he would submit draft constitutional amendments offering broader powers to the regions and voiced hope for quick approval by parliament. He said that the Russian parliament's move to rescind the sanction for the use of force was the result of his peace plan.
Ukraine and the West have accused Russia of supporting the rebellion with troops and weapons, but Moscow has denied that.
Putin's March 1 request to parliament for authority to use the military in Ukraine came days after Ukraine's pro-Russian president was chased from power following months of street protests. Russia sent troops that quickly overran Ukraine's Black Sea peninsula of Crimea, setting the stage for Russia to annex it after a hastily called referendum. In April, a mutiny erupted in eastern Ukraine, where pro-Russian insurgents seized official buildings, declared their regions independent and fought government troops.
John-Thor Dahlburg in Brussels and David Rising in Berlin contributed to this report.
Copyright 2014 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.