"Listen to the sound of the ceasefire," joked one armed rebel. "There's a proper battle going on there."
The two cities then turned quiet for much of Sunday, but a Reuters witness in the early evening reported several mortar blasts within the city confines of Donetsk. They damaged a bridge where the rebels had erected a roadblock.
In a new report on the conflict, Amnesty International accused both the rebels and Ukrainian militia of war crimes and it published satellite images it said showed a build-up of Russian armour and artillery in eastern Ukraine.
"Our evidence shows that Russia is fueling the conflict, both through direct interference and by supporting the separatists in the east. Russia must stop the steady flow of weapons and other support to an insurgent force heavily implicated in gross human rights violations," Amnesty's secretary-general, Salil Shetty, said in a statement.
Moscow denies dispatching forces or arming the rebels despite what NATO says is overwhelming evidence to the contrary.
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Western arms - or not?
Poroshenko spent Thursday and Friday at a NATO summit in Wales at which U.S. President Barack Obama and other leaders urged Putin to pull forces out of Ukraine. NATO also approved wide-ranging plans to boost its defences in Eastern Europe in response to the Ukraine crisis.
A senior aide to Poroshenko, Yuri Lytsenko, wrote on his Facebook page on Sunday that Kiev had reached agreement at the summit on receiving weapons and military advisers from five allies - the United States, France, Italy, Poland and Norway.
He gave no further details, but four of the five countries denied offering such assistance.
A senior Obama administration said the United States "has not changed policy" toward Ukraine, which thus far has been to provide only non-lethal military assistance.
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The official, who spoke on condition of anonymity, did not appear to rule out the possibility that Washington might at some point choose to offer arms to Kiev.
The official said the United States had so far given $70 million in security assistance to Ukraine and noted that U.S. President Barack Obama had said he was "looking at what more we can do."
Officials in Italy, Norway and Poland issued similar denials.
"Italy, along with other EU and NATO countries, is preparing a package of non-lethal military aid such as bullet-proof vests and helmets for Ukraine," an official at Italy's Defence Ministry said.
In France, an aide at the president's Elysee Palace declined to comment.
Read MoreQuestions surroundeastern Ukraine cease-fire
NATO officials have said the alliance will not send arms to non-member Ukraine, but they have also said individual allies may do so if they wish. A NATO official contacted by Reuters on Sunday about the Lytsenko comment reiterated that line.
Russia is fiercely opposed to closer ties between Ukraine and the NATO alliance.
The Ukraine conflict has revived talk of a new Cold War as the West accuses Putin of deliberately destabilising the former Soviet republic of 46 million people. Ukraine's prime minister accused Putin of striving to recreate the Soviet Union.
Putin says he is defending the interests of ethnic Russians facing discrimination and oppression in Ukraine since protesters toppled Kiev's pro-Russian president in February.
Putin has seen his popularity in Russia soar since Moscow annexed Ukraine's Black Sea peninsula of Crimea, which has a Russian majority, in March.
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Both the rebels and the Ukrainian military insisted on Sunday they were strictly observing the ceasefire and blamed their opponents for any violations.
"As far as I know, the Ukrainian side is not observing the ceasefire. We have wounded on our side at various points. We are observing the ceasefire," Vladimir Antyufeyev, deputy premier of the self-proclaimed "Donetsk People's Republic", told Reuters.
Earlier, government forces said they had come under artillery fire east of Mariupol, a crucial port for Ukrainian steel exports. In the days before the ceasefire, they had been trying to repel a big rebel offensive against the city.
The shelling in Mariupol claimed the first civilian casualty since the ceasefire began. Local officials confirmed the death of a 33-year-old woman early on Sunday and said at least four other people had been wounded.
Read MoreUkraine cease-fire: Sunset on conflict or false dawn?
"They, terrorists, Russians, are trying to scare us. They have no respect for the ceasefire. They are lying all the time. They are people with no honor," said Slavik, a Ukrainian soldier armed with a machine gun.
A Reuters reporter at the scene, a few km (miles) from the center of the city of 500,000, saw fires raging just before midnight on Saturday as Ukrainian reinforcements raced east towards the demarcation line separating the two sides.
Poroshenko agreed to the ceasefire after Ukraine accused Russia of sending troops and arms onto its territory to bolster the separatists after they suffered heavy losses over the summer to a Ukrainian government offensive.
The peace roadmap agreed on Friday includes an exchange of prisoners of war and the establishment of a humanitarian corridor for refugees and aid. There was no sign of progress on either plan on Sunday.
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