A woman was killed and four people injured in that shelling.
Early on Monday, a Reuters reporter heard a short blast of renewed artillery fire to the east of Mariupol but the area later fell quiet.
Both the rebels and the Ukrainian military insist they are strictly observing the ceasefire and blame their opponents for any violations.
The Ukrainian military's press centre listed five rebel violations of the ceasefire accord overnight into Monday, while the separatists accused government forces of preparing to storm a town near rebel-held Donetsk, the region's industrial hub.
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A Reuters reporter heard renewed mortar fire in the vicinity of the airport north of Donetsk around midday on Monday.
Handover of POWs
Russian Prime Minister Dmitry Medvedev said Moscow would respond to new sanctions over Ukraine, possibly targeting flights over Russia, if its Western partners press on with "the temptation to use force in international relations".
The European Union has agreed on new economic sanctions, including oil company Rosneft and units of gas producer Gazprom, which should come into force on Monday, but said they could be suspended to allow Moscow to show it was resolving the conflict.
Ukrainian media said the rebels had handed over 15 prisoners-of-war on Sunday, though this could not be confirmed.
"Originally it was proposed (under the peace roadmap) that exchanges would take place on Wednesday and we are preparing for that ... We will exchange all for all," Russia's Interfax news agency quoted senior separatist leader Andrei Purgin as saying.
In rebel-held Donetsk, the largest city of eastern Ukraine with a pre-war population of about one million, authorities of the self-proclaimed "Donetsk people's republic" declared Monday a public holiday to mark the expulsion of "fascists" from the heavily industrialised, mainly Russian-speaking Donbass region.
Read MoreUkraine's Poroshenko says close to point of all-out war with Russia
The separatists have used the word "fascist" to denote the central government in Kiev since Poroshenko's predecessor Viktor Yanukovich, who came from eastern Ukraine and was backed by Mosocw, fled to Russia in February after months of anti-government protests in the Ukrainian capital.
Most residents of Donetsk blame Kiev for the conflict, especially after months of heavy bombardment of the city by government forces, but some had harsh words also for the separatists and few expected the current ceasefire to last.
"The ceasefire is not holding, that's clear from just the few days I have been back in the city," said Yevgenia, who has taken refuge with relatives in western Ukraine.
"We came back for warm clothes and are leaving right away. It's so sad to see the city empty, deserted, armed people with cars crossing at red traffic lights, kidnapping people or taking away their cars. What good can they build here?"
Rights group Amnesty International, in a report published on Sunday, accused both sides in the conflict of war crimes and also said Russia was deliberately fueling the violence by direct interference and by its support for the rebels.
Read MoreNATO: Russia 'attacking'Ukraine as chasm widens
Moscow denies sending troops into Ukraine or arming the rebels, despite what Kiev and its Western backers say is overwhelming evidence to the contrary.
Ambassador Greminger said there were plans to double the OSCE monitoring mission to 500 observers. Swiss Foreign Minister Didier Burkhalter said talks were underway on the possibility of using "unmanned aerial vehicles", also known as drones.
The new U.N. High Commissioner for Human Rights, Zeid Ra'ad al Hussein, said in his maiden speech in Geneva on Monday: "Every effort must be made by both the Ukrainian government and the armed groups - as well as by neighbouring states, including the Russian Federation - to protect civilians in this deplorable conflict, and to ensure compliance with international law."
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