Ukraine crisis: Ceasefire is 'largely holding'
The Ukrainian and Russian presidents have said that a ceasefire in eastern Ukraine is "largely holding".
Ukraine's Petro Poroshenko and his Russian counterpart Vladimir Putin spoke by phone to discuss steps to make Friday's truce durable.
But by Saturday evening there were reports of shelling near the southern city of Mariupol.
Meanwhile, Russia vowed to respond if the European Union imposed new sanctions over the Ukraine crisis.
The EU says the sanctions, targeting more Russian individuals, will be introduced on Monday but could be later suspended if Russia withdraws troops from eastern Ukraine and observes a current truce.
Russia has repeatedly denied accusations by Ukraine and the West that it has been sending regular troops into eastern Ukraine to help the rebels.
Some 2,600 people have died in fighting after pro-Russian rebels seized towns in Ukraine's eastern Donetsk and Luhansk regions in April - a month after Russia's annexation of the southern Crimean peninsula.
In a statement Mr Poroshenko also said that the two presidents had stressed the need "to maximise the involvement" of the Organization for Security and Co-operation in Europe (OSCE) in monitoring the truce.
The two leaders also discussed ways of co-operating in delivering humanitarian aid to the region.
In his turn, President Putin said in a statement that an agreement was reached to "continue dialogue".
The ceasefire deal was signed during talks between representatives of Ukraine, Russia, the OSCE and the separatist rebels in Minsk, Belarus.
The truce came into effect at 15:00 GMT on Friday.
There were no reports of major fighting in the east for the first 24 hours, but by Saturday evening the BBC's Fergal Keane tweeted from Mariupol that shelling had resumed.
The spokesman for Ukraine's National Security and Defence Council, Andriy Lysenko, said earlier on Saturday that the rebels had fired 10 times on Ukrainian troops since the truce.
Unconfirmed reports also say a number of fighters from Ukraine's Aydar battalion were ambushed and killed after the ceasefire.
Meanwhile, the rebel leader of the self-proclaimed Donetsk People's Republic, Aleksandr Zakharchenko, said the truce was "not being fully observed" and that rebels had been subjected to shelling in the town of Amvrosiyivka near Donetsk.
Also the International Committee of the Red Cross said in a tweet that aid trucks on their way to Lugansk had been forced to turn back by shelling, without giving any further details.
Our correspondent also spoke to Andriy Biletskiy, the commander of Ukraine's Azov battalion, who said he believed fighting would resume within "five to seven days".
"We will see how the situation develops," he said. "If it was a tactical move there is nothing wrong with it... if it's an attempt to reach an agreement concerning Ukrainian soil with separatists then obviously it's a betrayal."
A BBC crew that travelled to Donetsk airport on Saturday morning heard a few gunshots and small explosions but residents said the night had been quiet.
'Party of war'
Earlier on Saturday, the Russian foreign ministry said there "will undoubtedly be a reaction from our side" if the new EU sanctions were passed.
The fresh sanctions would add another 24 to the list of people barred from entering the EU and whose assets have been frozen.
Among them would be the rebel leadership in the eastern Donetsk and Luhansk regions, officials in Crimea, which was annexed by Russia in March, and Russian "decision-makers and oligarchs", European Council President Herman Van Rompuy and European Commission chief Jose Manuel Barroso said in a statement.
The Russian foreign ministry said the EU was "practically sending a signal of direct support to the 'party of war' in Kiev".