A woman reacts as anti-government protesters place a dead body on a stretcher after violence erupted in the Independence Square in Kiev February 20, 2014. REUTERS/Vasily Fedosenko

KIEV (Reuters) - Fresh fighting broke out in central Kiev on Thursday, shattering a truce declared by Ukrainian President Viktor Yanukovich, as the Russian-backed leader met European ministers demanding he compromise with pro-EU opponents.

A Reuters photographer saw the bodies of 21 dead civilians in Independence Square, a few hundred meters (yards) from where the president met the EU delegation, after protesters who have occupied the area for almost three months hurled petrol bombs and paving stones to drive riot police out of the plaza.

Buzzfeed reports that at least 35 have died.

The foreign ministers of Germany, France and Poland were to report back in Brussels later in the day to EU colleagues, who will decide on possible targeted sanctions against those deemed responsible for the bloodshed.Russia criticized the European and U.S. measures, saying they would only make matters worse.

A Ukrainian presidential statement said dozens of police were killed or wounded during the opposition offensive hours after Yanukovich and opposition leaders had agreed on a truce. Witnesses said they saw snipers firing during the clashes. The Health Ministry said two police were among Thursday's dead.

That raised the total death toll since Tuesday to at least 51, including at least 12 police - by far the bloodiest hours of Ukraine's 22-year post-Soviet history. Local media said more than 30 protesters were killed in Thursday's flare-up.

The country is the object of a geopolitical tug-of-war between Moscow, which sees it as the historical cradle of Russian civilization, and the West, which says Ukrainians should be free to choose economic rapprochement with the EU.

The renewed fighting, which subsided after about an hour, heightened concern voiced by neighboring Polish Prime Minister Donald Tusk that Ukraine could descend into civil war or split between the pro-European West and Russian-speaking east.

The EU ministers' meeting with Yanukovich was delayed for security reasons but began an hour late. They expected to present him with a mixture of sanctions and enticements to make a deal with his opponents that could end the bloodshed.

A statement from Yanukovich's office said: "They (the protesters) are working in organized groups. They are using firearms, including sniper rifles. They are shooting to kill.

"The number of dead and injured among police officers is dozens," the statement on the presidential website said.

Opposition leader Vitaly Klitschko said on Sky TV: "As we can see the truce has broken."

Television showed activists in combat fatigues leading several captured, uniformed policemen across the square.

POLICE OFFICERS CAPTURED

Activists who recaptured the square, known as Maidan or "Euro-Maidan" to the opposition, appeared to lead away several uniformed officers. Dozens of wounded protesters were being given makeshift first aid treatment in the lobby of the Ukraine Hotel, where many foreign correspondents are staying.

Reporters said there were bullet holes in the walls and windows of the hotel overlooking the square. Both sides have accused the other of using live ammunition.

"Black smoke, denotations and gunfire around presidential palace ... Officials panicky," tweeted Polish ministerRadoslaw Sikorski to explain the delay in the meeting at Yanukovich's office, a few hundred meters from the square.

Pro-EU activists have been keeping vigil there since the president turned his back on a trade pact with the bloc in November and accepted financial aid from Moscow.

Russia, which has been holding back a new loan installment until it sees stability in Kiev, has condemned EU and U.S. support of the opposition demands that Yanukovich, elected in 2010, should share power and hold new elections.

In an apparent criticism of Yanukovich's handling of the crisis, Russian Prime Minister Dmitry Medvedev said on Thursday that Moscow could only cooperate fully with Ukraine when its leadership was in "good shape", Interfax news agency said.

The crisis in the sprawling country of 46 million with an ailing economy and endemic corruption has mounted since Yanukovich, under pressure from the Kremlin, took a $15-billion Russian bailout instead of a wide-ranging deal with the EU.

The United States stepped up pressure on Wednesday by imposing travel bans on 20 senior Ukrainian officials, and European Union foreign ministers are due to meet in Brussels later on Thursday to consider similar measures.

A statement on Yanukovich's website announced an accord late on Wednesday with opposition leaders for "the start to negotiations with the aim of ending bloodshed, and stabilizing the situation in the state in the interest of social peace".

Responding cautiously, U.S. President Barack Obama deemed the truce a "welcome step forward", but said he would monitor the situation closely to "ensure that actions mirror words".

"Our approach in the United States is not to see these as some Cold War chessboard in which we're in competition with Russia," Obama said after a North American summit in Mexico.

At Russia's Winter Olympics in Sochi some members of Ukraine's team have decided to leave because of the violence at home, the International Olympic Committee said on Thursday.

COMBATIVE MOOD

Protesters were in a truculent mood despite the overnight lull and columns of men, bearing clubs and chanting patriotic songs headed to Independence Square at 8:30 a.m. (0130 ET).

"What truce? There is no truce! It is simply war ahead of us! They are provoking us. They throw grenades at us. Burn our homes. We have been here for three months and during that time nothing burned," said 23-year-oldPetro Maksimchuk.

"These are not people. They are killers. Sanctions will not help. They all should be sent into isolation in Siberia."

Serhiy, a 55-year-old from the western city of Lviv who declined to give his surname, added: "It is bad that Ukraineis already broken into two parts. In the west the police and army are with us but in the east, they are against us.

"It is the 'Yanukovichers' who are dividing us."

In Lviv, a bastion of Ukrainian nationalism since Soviet times, the regional assembly declared autonomy from Yanukovich and his administration, which many west Ukrainians see as much closer to Moscow and to Ukraine's Russian-speaking east.

Yanukovich, who replaced the head of the armed forces, had denounced the bloodshed in central Kiev as an attempted coup. His security service said launched a nationwide "anti-terrorist operation" after arms and ammunition dumps were looted.

The EU ministers were expected to consider a series of possible steps including asset freezes and travel bans, even though diplomats doubt the are effective.

Jumping out ahead of its EU allies, Washington imposed U.S. visa bans on 20 government officials it considered "responsible for ordering human rights abuses related to political oppression", a State Department official said.

"These individuals represent the full chain of command we consider responsible for ordering the security forces to move against" the protesters, the official said.

EU officials said Yanukovich himself would be excluded from such measures in order to keep channels of dialogue open.

Diplomats said the threat of sanctions could also target assets held in the West by Ukrainian business oligarchs who have either backed Yanukovich or are sitting on the fence.

Russian President Vladimir Putin, who has met Yanukovich six times since the crisis began, has kept quiet on the flare-up. But Foreign Minister Sergei Lavrov blamed the West for inciting opposition radicals and called the threat of sanctions blackmail.

Ukraine's hryvnia currency, flirting with its lowest levels since the global financial crisis five years ago, weakened again on Thursday.

Possibly due to the risk of sanctions, three of Ukraine's richest magnates have stepped up pressure on Yanukovich to hold back from using force.

"There are no circumstances which justify the use of force toward the peaceful population," steel and coal magnate Rinat Akhmetov, who bankrolled Yanukovich's 2010 election campaign said in a statement late on Tuesday.

(Additional reporting by Natalya Zinets, Pavel Polityuk and Alessandra Prentice in Kiev; Writing by Richard Balmforth and Paul Taylor; Editing by Alastair MacDonald)