Pro-Russia rebels in eastern Ukraine say they will observe a ceasefire until Friday morning, responding to the Ukrainian forces' unilateral ceasefire.
The rebel announcement was made in Donetsk by Alexander Borodai, a leader of the self-styled "Donetsk People's Republic" which is defying Kiev.
On Friday, Ukrainian President Petro Poroshenko announced a 15-point peace plan and declared a week-long truce.
Mr Borodai was speaking after attending preliminary peace talks in Donetsk.
The high-level talks also involved representatives of the other breakaway region - Luhansk - and Viktor Medvedchuk, an opponent of the Kiev authorities who is close to Russian President Vladimir Putin.
Russia's Ambassador Mikhail Zurabov was also there, along with former Ukrainian President Leonid Kuchma, who is seen as a mediator for President Poroshenko. European mediators from the OSCE security organisation also participated.
Conditions of truce
Last week President Putin cautiously welcomed the Kiev ceasefire. The plan involves decentralising power, holding early elections, and creating a 10km (six-mile) buffer zone on the Ukrainian-Russian border.
The rebels say they will not disarm until Ukrainian government troops have left the east. The militants still control key government buildings across Donetsk and Luhansk regions.
"The ceasefire will take effect as announced earlier - until 10:00 (07:00 GMT) on 27 June," Mr Borodai said, referring to President Poroshenko's declaration on Friday. "During that time there will be ceasefire monitoring by Russia and the OSCE.
"We hope that during the ceasefire both sides will manage to reach agreement and start consultations on how to proceed with talks and a peace settlement."
Analysis by David Stern, BBC News, in Kiev
The announcement of a general ceasefire is just a first step towards ending the escalating conflict in Ukraine's east - but it's a significant development nonetheless.
Just one day ago, the pro-Russian insurgents rejected President Petro Poroshenko's declaration of a temporary truce, saying it was a ploy. Now they have also agreed to halt combat operations until Friday.
But there are still potential hazards that could derail the peace process. European officials, echoing their Ukrainian and American counterparts, say Russia is still allowing fighters and arms to cross its border with Ukraine.
Russian President Vladimir Putin for his part has given a conditional approval to Mr Poroshenko's peace plan, but says that Kiev must now negotiate directly with the rebels.
Mr Poroshenko has said he will not talk to terrorists.
Fighting could very well resume on Friday, if not earlier. But for the moment, the killing has stopped, and the warring sides have received enough breathing room to move forward. This in itself is an accomplishment.
The development followed talks on the Ukraine crisis between EU foreign ministers in Luxembourg with the new Ukrainian Foreign Minister, Pavlo Klimkin.
The Council of the European Union called on Russia "to adopt effective measures to stop the continued flow of illegal fighters, arms and equipment over the border into Ukraine" and "to use its influence on the separatists to stop the violence and lay down their arms," in a statement.
In a telephone call on Monday, US President Barack Obama also issued President Putin a warning that Russia would face new sanctions if it both failed to stop the flow of weapons into Ukraine and took no action to de-escalate the crisis.
The US and EU had already imposed a number of sanctions, including visa bans and asset freezes on Russian officials, after Moscow annexed the Ukrainian region of Crimea in March.
But so-called "Phase Three" sanctions would mean wider economic measures denying Russian access to certain technologies and investments.
The crisis is expected to be high on the agenda of an EU summit in Brussels on Friday, where European leaders could announce further sanctions against Russia if they deem Moscow's response to the peace plan to be inadequate.
Western leaders accuse Russia of arming the anti-Kiev militants in Donetsk and Luhansk, where ties with Russia are traditionally strong.
Friday will also see the signing of a key EU-Ukraine association agreement - a far-reaching partnership deal which lies at the origin of the Ukraine crisis.
The refusal of former Ukrainian President Viktor Yanukovych to sign the agreement last November, under heavy Russian pressure, triggered massive pro-EU street protests which forced him out of office.