* Heavy fighting near Mariupol port and in city of Donetsk
* Envoys expected to agree ceasefire and peace road map
* Residents and combatants sceptical about peace prospects (Adds conflicting claims over rebel entry into Mariupol)
By Aleksandar Vasovic and Gabriela Baczynska
MARIUPOL/DONETSK, Ukraine, Sept 5 (Reuters) - Fighting raged between Ukrainian forces and pro-Russian rebels just east of the strategic port of Mariupol on Friday despite the start of talks between envoys from Ukraine and Russia in Minsk on a ceasefire and a peace plan.
Sustained mortar and artillery fire were also heard in Donetsk, the rebels' main stronghold in eastern Ukraine, from near the city's airport, which remains in government hands.
Ukraine says its forces are trying to repel a big offensive by the rebels to take Mariupol, a port city of around 500,000 on the Sea of Azov crucial to Ukraine's steel exports. It stands about halfway between Russia and the Russian-annexed Crimean Peninsula.
Ukrainian President Petro Poroshenko said on Thursday he would order a ceasefire if negotiations began in Minsk between envoys from Ukraine, the pro-Russian separatist leadership, Russia and Europe's OSCE security watchdog.
But a Ukrainian officer told Reuters in Mariupol after the talks had started: "Our operations are continuing as scheduled until further orders."
Ukrainian officials and commanders said their forces were pounding rebel positions to the east of the city and denied separatist claims that their forces had entered Mariupol.
"(This claim) is not true. We have even driven them back from the positions they held before," said Andriy Lysenko, spokesman for Ukraine's National Security and Defence Council.
A spokesman for the rebels' self-proclaimed 'Donetsk People's Republic' (DNR) told Reuters: "The army of the DNR has partly entered Mariupol. But the city is not taken."
Mariupol became a major focus of concern for Ukraine after the rebels broke out of their main strongholds further north in late August - backed, Kiev says, by Russian regular forces.
Russia denies sending troops and weapons into Ukraine, despite what NATO says is overwhelming evidence to the contrary.
A Ukrainian military spokesman told a daily news briefing in Kiev that about 2,000 Russian servicemen had been killed so far in the Ukraine conflict. There was no way of confirming the figure independently. The United Nations recently put the total death toll in the conflict to date at more than 2,600.
"We have come for peace, the main thing is to get an armistice," said former Ukrainian president Leonid Kuchma, Kiev's representative at the Minsk talks.
However, few in eastern Ukraine, wearied by nearly six months of conflict, have much hope that a ceasefire can hold and some said it was a bad idea that would only benefit the enemy.
"A ceasefire would be a disaster, we would lose everything. By fighting, we can resist the invasion and send them back. With a ceasefire, they will consolidate and carry on after a while," said Taras, a Ukrainian soldier.
Another soldier who gave his name as Mykola said Poroshenko - who was attending the second day of a NATO summit in Wales - would be "betraying the country" if he backed a peace plan now:
"If he goes for a peace plan, then all these dead and wounded and exiled and all the homes burned and jobs lost and money lost, it was all for nothing."
In rebel-held Donetsk, where residents mostly tend to blame the Ukrainian side for the conflict, scepticism was also strong.
"I doubt Ukraine would go for any ceasefire. Poroshenko may announce it for the army but there are also the Kolomoisky battalions," said Denis Tikhinov, 22, who worked for a computer servicing firm before it shut because of the fighting.
He was referring to battalions formed by Ihor Kolomoisky, a wealthy businessman and governor of the Dnipropetrovsk region, to help fight the pro-Russian separatists.
"When the first ceasefire was announced by Poroshenko, Kolomoisky said there would be no such thing and gave orders to his battalions to go on destroying the city and civilians."
On Friday, people queued to get water from cisterns, to sign up for humanitarian aid and to withdraw money from the cash machine of one of the few banks still working in Donetsk, which had a pre-war population of about 1 million.
A mosque, shops and schools were among buildings damaged by the renewed shelling on Friday morning.
"I have no hope. Poroshenko is a traitor, he makes promises but he's just lying to the people," said pensioner Lidia. (Writing by Gareth Jones; additional reporting by Andrei Makhovsky in Minsk and Pavel Polityuk in Kiev; Editing by Kevin Liffey)
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