Russian President Vladimir Putin and Ukraine's President Petro Poroshenko agreed on Saturday in a telephone call that a ceasefire in eastern Ukraine was generally holding but said further steps were needed to make it more durable.
The ceasefire between Ukrainian government forces and pro-Russian separatists took effect on Friday evening, part of a wider peace plan aimed atmending five months of fighting in eastern Ukraine.
"(The two leaders) also stressed the need for the maximum involvement of the Organisation for Security and Cooperation in Europe (OSCE) in monitoring the situation ... and for cooperation in providing Ukrainian and international humanitarian help," Poroshenko's office said in a statement.
Earlier, Russia's Foreign Ministry vowed to react if the European Union implements new sanctions against Moscow over its role in the Ukraine crisis.
The EU announced the additional measures late on Friday but said they could be suspended if Moscow withdraws its troops from Ukraine and observes a newly agreed ceasefire in eastern Ukraine.
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The new sanctions include adding a further 24 people to a list of people barred from entry to the 28-nation bloc and whose assets are frozen. They are due to be implemented on Monday.
"If they (new sanctions) are implemented of course there will be a reaction from our side," the ministry said in a statement.
Residents and combatants said the cease-fire was likely to prove a brief interlude before renewed fighting.
Underlining the fragility of the truce, approved on Friday by envoys from Ukraine, the separatist leadership, Russia and the OSCE security watchdog, some residents in rebel-held Donetsk reported sporadic shelling overnight on the city outskirts.
"It is looking good for now but we know they (the Ukrainian side) are only using it to bring in more forces here and ammunition and then to hit us with renewed strength," said one rebel commander known by his nickname Montana.
"Come what may, I would not trust (Ukraine's President Petro) Poroshenko.
And it's not him making the call anyway but the Americans and that is even worse."
Poroshenko agreed to the ceasefire after Ukraine accused Russia of sending troops and arms onto its territory in support of the separatists, who had suffered big losses over the summer. Moscow denies sending troops or arming the rebels.
The truce has raised hopes of an end to fighting that has caused the worst standoff between Moscow and the West since the Cold War ended. But U.S. President Barack Obama, who has also accused Russia of involvement, said on Friday he was sceptical the separatists would follow through.
A Donetsk resident, Ksenia, said she had heard some shooting overnight after the ceasefire had come into effect.
"I don't know what ceasefire we are talking of if there was shooting again.
This is no ceasefire but a theatre. This war will go on for five to nine years. Slavs are killing Slavs, there can be nothing worse than that," she said.
Dmytro Tymchuk, a defense analyst with links to government forces, accused the rebels of violating the ceasefire at various sites around the region, adding: "The Ukrainian forces are fully observing the conditions of the ceasefire and only open fire when their positions come under attack."
All was quiet in and around the port of Mariupol on the Sea of Azov, the scene of fierce fighting on Friday.
"Many of my men had their first good sleep in days," said one Ukrainian army officer. "I certainly slept well."
Fighting had raged in recent days both on the outskirts of Donetsk, especially near the airport, which remains in government hands, and to the east of the port of Mariupol, where government forces were holding off a major rebel offensive.
The peace plan approved by envoys in Minsk on Friday also includes an exchange of prisoners and the establishment of a humanitarian corridor for refugees and aid.