* Heavy fighting near Mariupol port and in city of Donetsk
* Envoys expected to agree ceasefire, peace road-map
* Residents, combatants very sceptical about peace (Adds new details, quotes from Mariupol, Donetsk)
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, shortly before envoys from Ukraine and Russia were expected to announce a ceasefire as the starting point for a wider 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 for its steel exports. It stands about halfway between Russia and the Russian-annexed Crimea region.
"Our artillery has come and is being deployed against the rebels," said the mayor of Mariupol, Yuri Khotlubey.
The commander of the Azov volunteer militia, Andriy Biletsky, said his men had regained territory from the rebels in a counter-offensive after they came within just five km (three miles) of Mariupol on Thursday.
"We brought sufficient artillery and reinforcements," he told Reuters at one of the checkpoints erected on the outskirts of the port to defend it from the rebels.
Mariupol became a major focus of concern for Ukraine after the rebels broke away from 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 independently confirming the figure. The United Nations recently put the total death toll in the conflict to date at more than 2,600.
A Ukrainian army doctor showed the fragment of a shell he said had killed a woman and two children in a village near Mariupol. "It was a Russian rocket, we know it from its markings," he said.
Representatives from Ukraine, the pro-Russian rebel leadership, Russia and Europe's OSCE security watchdog arrived in the Belarussian capital Minsk to sign around 1100 GMT a peace plan for the region that will impose a ceasefire by both sides.
"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 Ukrainian soldier Taras.
Another Ukrainian soldier who gave his name as Mykola said Ukrainian President Petro Poroshenko - who was attending the second day of a NATO summit in Wales on Friday - would "betray the country" if he backed a peace plan at this time.
"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," he said.
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 Kolomoiksy, 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 one million.
A mosque, shops and schools were among buildings damaged by the renewed shelling on Friday morning.
"I have no hopes. Poroshenko is a traitor, he makes promises but he's just lying to the people," said pensioner Lidia. (Writing by Gareth Jones, editing by; additional reporting by Andrei Makhovsky in Minsk and Pavel Polityuk in Kiev; editing by Ralph Boulton)