KIEV (AFP) - Ukraine's president said on Friday a deal had been clinched to end a three-month-old standoff with opposition protesters after the worst day of violence left more than 60 dead and turned central Kiev into a war zone.

Police opened fire with Kalashnikovs on groups advancing behind makeshift shields while opposition medics said government snipers picked off protesters from rooftops on Thursday, in scenes that sparked global alarm.

Envoys from Germany, Poland and France - who negotiated through the night with Mr Viktor Yanukovych - warned, however, that the deal was not yet definitive.

"The parties agreed on the initialling of an agreement to resolve the crisis," the presidency said in a statement after the night of talks that also included a Russian envoy.

"The signing is expected at 12:00 (6:00 pm Singapore time)" at the presidency, the statement said, without giving any details on the deal.

According to the privately-owned Ukrainian 1+1 television channel, the agreement would see the country revert to the 2004 constitution within 48 hours, which would give more power to parliament and government, and less to the president.

A coalition government would also be formed within 10 days, and early presidential polls held in December - all key opposition demands.

But there was no immediate confirmation of a deal from any of the country's top three opposition leaders and EU envoys said negotiations were due to resume again at midday.

French Foreign Minister Laurent Fabius cautioned that the agreement was not "definitive", while a German delegation source said there was no final agreement.

The crisis in Ukraine flared in November when Mr Yanukovych declined to sign an EU integration deal in favour of closer ties with historical master Russia, and has evolved into a Cold War-style standoff between Moscow and the West over the future of the strategic nation sandwiched between them.

The violence of the past few days prompted the European Union to agree to impose sanctions on Ukrainians with "blood on their hands".

The United States also threatened to follow suit.

German Chancellor Angela Merkel, meanwhile, spoke to Presidents Barack Obama of the United States and Vladimir Putin of Russia - who have bickered openly over the crisis - by telephone.

All three called for a halt to the bloodshed that has escalated since Tuesday.

Kiev authorities put the death toll of the past few days at 77, but opposition medics say more than 60 protesters were shot dead by police on Thursday alone.

'SHOT IN HEAD OR HEART'

As such, the deal - if confirmed - could fail to satisfy protesters still reeling from the violence.

On Thursday, masked protesters forced gun-toting riot police from the capital's Independence Square - claiming back what has become the epicentre of an increasingly bloody revolt against Yanukovych's pro-Russian rule.

Armed with Molotov cocktails, batons and paving stones, they pushed back the feared Berkut police that had brutally reconquered the square on Tuesday night.

Police responded with rubber bullets, stun grenades but also with Kalashnikov rifles.

The White House said bluntly that it was "outraged by the images of Ukrainian security forces firing automatic weapons on their own people".

Volunteer medics, who made a makeshift morgue out of a popular hotel overlooking the square, also accused police of killing demonstrators with live rounds.

"They were shot in the head or in the heart by live bullets, not by rubber ones," said first aid worker Natalia.

Ukraine's interior ministry said only that it reserved the right to use live munitions "in self-defence".

The ministry also accused "extremists" of seizing 67 of its troops at gunpoint and holding them hostage in one of the buildings near the war-scarred square.

EU SANCTIONS

The shocking scale of bloodshed in a strategic nation of 46 million sandwiched between Russia and the European Union prompted EU officials to slap travel bans against Ukrainians responsible for ordering the use of force.

Italian Foreign Minister Emma Bonino said an agreement was also reached at an emergency EU meeting in Brussels to impose asset freezes on those with "blood on their hands".

The measures mark a U-turn for Brussels diplomats, who until Monday had resisted Ukrainian opposition demands for sanctions.

Vice President Joe Biden on Thursday warned Mr Yanukovych that the United States was ready to impose sanctions on officials guilty of ordering troops to fire on protesters.

Washington has already put 20 top Ukrainian officials on a visa blacklist.