Donetsk, Ukraine: Ukraine on Tuesday pressed on with a punishing offensive to win back its war-torn east, after claiming that pro-Russian rebels had killed “dozens” of fleeing civilians in a rocket strike.

Kiev said “street battles” with insurgents had erupted in the heart of the rebel bastion of Lugansk, which has endured brutal shelling and weeks without running water or electricity as Ukrainian forces seek to roust separatists there.

The military on Monday accused the insurgents of using Russian-supplied missiles to blow up adults and children who were attempting to escape the besieged city in a convoy flying white flags. The attack left “dozens of dead”, according to Ukraine’s security spokesman Andriy Lysenko.

Fifteen bodies were recovered after the strike on the convoy, Lysenko said on Tuesday.

The rebels denied the allegations, which could not be independently verified.

But the claim of a strike on civilians drew calls for restraint from the United Nations and Washington, after weekend talks between Russian and Ukrainian top diplomats failed to make any breakthroughs.

UN Secretary General Ban Ki-moon said the “tragic incident makes the urgency of a ceasefire and a diplomatic solution even more stark.”

Ban urged both Kiev forces and rebels to “allow safe passage to anyone attempting to leave areas of active military operations,” a spokesman said.

The State Department in Washington said it had been unable to confirm who was responsible for the attack on the convoy and urged “all sides [to] take every precaution to protect innocent lives.”

Four months of fighting in eastern Ukraine has left more than 2,100 dead and a brewing humanitarian crisis in the region.

Lugansk, still in rebel hands, has been hardest hit. Water and power have been cut off for more than two weeks, and food is becoming increasingly scarce.

Ukrainian army sources said on Tuesday “street battles” had moved into the centre of the city after one rebel-held district had been “liberated”.

If confirmed, any advance by government forces into the centre of the industrial hub, once numbering some 420,000, would represent a major breakthrough for Kiev.

In the main insurgent stronghold of Donetsk, meanwhile, an AFP reporter heard the rumble of explosions around the city as government troops pounded rebels positions there.

Queuing for water

Residents in that city, which had a pre-war population of one million, were again queuing for water after fighting cut supplies over the weekend.

An AFP photographer in the adjoining city of Makiyivka saw the bodies of one woman and two men killed by shelling sprawled in the streets.

Late on Monday, President Petro Poroshenko said Ukraine was readjusting its military strategy following fresh rebel claims they were receiving troop reinforcements from Russia to prop up their ailing insurgency.

Poroshenko said government forces were “regrouping” as they sought to “continue the offensive”.

More than 285,000 people have been forced to flee the fighting in eastern Ukraine and two senior UN officials — Under-Secretary General Jeffrey Feltman and humanitarian aid chief Valerie Amos — are set to travel to Kiev later this week.

Kiev said it was also looking forward to a “very interesting visit” by German Chancellor Angela Merkel on Saturday, one day before Ukraine celebrates Independence Day.

Diplomatic efforts to defuse the fighting continued despite a crisis meeting between the foreign ministers of Ukraine, Russia, Germany and France on Sunday having broken up without any agreement on how to end the violence.

Ukraine’s top diplomat Pavlo Klimkin lashed out at Russia for refusing “to recognise the facts” of the continuing flow of weapons and mercenaries across its border.

Russia has consistently denied Western allegations that it is funnelling weapons to the rebels and instead called on Kiev’s forces to halt firing.

A controversial Russian aid convoy was meanwhile still stuck waiting to be checked near Ukraine’s restive border as haggling dragged on about letting it across.

Red Cross representative Laurent Corbaz is due in Moscow on Tuesday to discuss with Russian officials the delivery of humanitarian aid to east Ukraine.

The Red Cross — which is meant to oversee the delivery of the cargo — says they have not yet received security guarantees on how it will cross rebel territory.

“We have no date, no hour” for when the convoy may go to the Ukrainian side, Paul Picard, a monitor for the OSCE at the border, told journalists.

Kiev and the West fear the shipment is a ploy to bolster the rebellion or provide a pretext for Russia to invade, allegations dismissed by Moscow.