MARIUPOL, Ukraine, Sept 07, 2014 (AFP) - Gunfire and heavy shelling rocked a key frontline city in eastern Ukraine overnight, raising fears Sunday that a tenuous truce between government and rebel forces had already collapsed.

Numerous explosions rattled the night sky and thick smoke was visible on the horizon of Mariupol, a strategic government-held port city on the Azov Sea in the southeast of the country.

Residents spoke of their panic as the fighting erupted, with gunfire and shelling that damaged buildings and vehicles.

“Everyone is starting to flee,” one 46-year-old Mariupol resident who gave her name only as Victoria told AFP.

“I’m frightened. I want peace but I think this ceasefire is finished, this is the third night we haven’t been able to sleep.”

The renewed violence erupted just hours after a phone call between Ukraine’s President Petro Poroshenko and his Russian counterpart Vladimir Putin, who agreed that the ceasefire signed on Friday was “generally being observed”.

The 12-point pact was the first to gain the backing of both Kiev and Moscow after five months of fighting that has claimed around 2,800 lives and triggered the deepest crisis in East-West relations for a generation.

It was drawn up after the rebels -- reportedly backed by Russian troops and firepower -- launched a lightning counter-offensive across the southeast in late August that dramatically reversed recent gains by the Ukrainian army.

Mariupol became the latest flashpoint when the insurgents pushed southwards in what is seen as a drive to carve out a land corridor between the Russian border and the strategic Crimean peninsula annexed by Russia in March.

The fresh violence threatened a repeat of the unilateral ceasefire called by Kiev in June, which collapsed within days.

Both sides were already accusing each other on Saturday of breaching the truce within hours of its signing in the Belarussian capital Minsk.

And pro-Russian separatists opposed to Kiev’s rule still insist they will not give up their ambitions for an independent state in the industrial east.

“We want our own president, our own currency and our own banking system,” a pro-Russian guerrilla named Oleg told AFP in the Donetsk region town of Yasynuvata.

“This is the only way. There is no other alternative.”

Western leaders accuse Russia of actively fomenting the rebellion by funneling large numbers of troops and heavy weaponry across the border -- claims which Moscow has repeatedly denied.

Despite the ceasefire, the US and the EU agreed to beef up sanctions against Russia, and NATO approved a rapid reaction force aimed at reassuring jittery Eastern European states.

Russia warned it would respond if the EU imposes more sanctions, accusing Brussels of supporting the “party of war” in Kiev.

“Instead of feverishly searching for ways to hurt the economies of its own countries and Russia, the European Union would do better to work on supporting the economic revival of the Donbass region” of eastern Ukraine, its foreign ministry said on Saturday.

The Minsk accord calls on both sides to start pulling back from major flashpoints and exchanging prisoners, as well as the supply of humanitarian aid to the devastated cities of east Ukraine.

There was no confirmation yet if any of the articles had yet been implemented.

Although Poroshenko said he was “satisfied” with the agreement, it opened him up to accusations that he has surrendered to recent rebel advances and failed to reunify the nation of 45 million under a pro-Western banner, as he promised at the time of his election in May.

The peace pact could leave the separatists -- who remain deeply mistrustful of the nationalist-leaning government in Kiev -- in effective control of a region that accounts for one-sixth of Ukraine’s population and a quarter of its exports.

The months of fighting have left dozens of towns in the east in ruins, and once-powerful factories and coal mines that form the backbone of Ukraine’s economy have ground to a halt.

“It’s impossible to trust them (the rebels), they are bandits,” said Natalia, a 54-year-old professor staying with friends in Mariupol after fleeing Donetsk.

A Human Rights Watch report on Saturday accused pro-Russian rebels of committing “serious violations of the laws of war”, claiming they were forcing civilians to work in “punishment brigades” on pain of death.

An Amnesty report also accused both sides of war crimes, including indiscriminate shelling, abductions, torture, and killings.

But despite strong rhetoric, there appears to be little appetite in Western capitals to become directly involved in ensuring the peace in the former Soviet state.

“This obviously is a ceasefire that has to be held between Russia and Ukraine,” U.S. State Department spokeswoman Marie Harf said.

“This isn’t about the United States; this is about them.”