* EU ministers hold "difficult night" of talks
* Presidency says deal reached, opposition silent
* Death toll in two days violence reaches 75 (Updates with statement, talks end)
By Pavel Polityuk and Sabine Siebold
KIEV, Feb 21 (Reuters) - Ukraine's presidency said on Friday a deal had been reached at all-night talks on resolving the country's crisis after 75 people were killed in two days of the worst violence since Soviet times.
But the opposition did not immediately confirm agreement had been reached and diplomatic sources described the talks brokered by three European Union ministers as "very difficult".
Three hours of fierce fighting on Thursday in Kiev's Independence Square, which was recaptured by protesters demanding President Viktor Yanukovich quit, left the bodies of civilians strewn on the ground close to where talks took place.
A few thousand people were occupying the square on Friday morning but there was no sign of any new violence after Thursday's bloodshed in Kiev, which is caught in a geopolitical tug-of-war between East and West.
Yanukovich's press service said in a statement that agreement had been reached at the talks and added: "The agreement will be signed at midday (1000 GMT)."
The three EU ministers, from Poland, Germany and France, did not immediately comment on the presidency statement.
EU officials went into the talks on Thursday hoping a plan for an interim government and early elections could bring peace.
France's foreign minister said late on Thursday there was still no agreement over a proposed road map to ease the crisis, which began in November after Yanukovich spurned a trade deal with the European Union and turned instead towards Moscow.
Shortly before the presidency statement was released, the Standard & Poor's agency lowered Ukraine's credit rating, saying the future of the country's leadership looked more uncertain than at any time since the crisis began. This could affect the delivery of financial aid promised by Russia, it said.
The violence has hit the Ukrainian currency, the cost of insuring the country's debt has risen and a senior general dealt Yanukovich another blow by tendering his resignation over the bloodshed, saying he feared more.
On Thursday, riot police were captured on video shooting from a rooftop at demonstrators in the central plaza, also known as the Maidan. Protesters hurled petrol bombs and paving stones to drive the security forces off a corner of the square the police had captured in battles that began two days earlier.
The health ministry said 75 people had been killed since Tuesday afternoon, which meant at least 47 died in Thursday's clashes. That was by far the worst violence since Ukraine emerged from the crumbling Soviet Union 22 years ago.
Yanukovich's position was looking increasingly difficult, especially after the resignation of Lieutenant-General Yuri Dumansky, deputy head of the armed forces general staff.
"The armed forces of Ukraine are being drawn into a civil conflict. This could be the cause of a large number of deaths of civilians and servicemen," Dumansky told Channel 5 television. "I have decided to tender my resignation to avoid an escalation and bloodshed."
Standard & Poor's said the political situation in Ukraine had deteriorated substantially and this raised uncertainty about the financial aid promised by Russia under a $15 billion bailout package needed to help Kiev repay huge debts.
The second instalment, of $2 billion, is expected to be paid soon but Moscow has signalled that Yanukovich must first restore order in the country of 46 million to get it.
"We consider that the future of the current Ukrainian leadership is now more uncertain than at any time since the protests began in November 2013," the ratings agency said.
The trio of visiting foreign ministers met Yanukovich and the opposition after EU colleagues in Brussels imposed targeted sanctions on Ukraine and threatened more if the authorities failed to restore calm.
In further diplomatic efforts, U.S. President Barack Obama spoke to German Chancellor Angela Merkel, who in turn discussed Ukraine with Russian President Vladimir Putin.
The White House said Obama and Merkel agreed it was "critical" U.S. and EU leaders "stay in close touch in the days ahead on steps we can take to support an end to the violence and a political solution that is in the best interests of the Ukrainian people". Earlier this month, bugged and leaked diplomatic phone calls exposed EU-U.S. disagreement on Ukraine.
Yanukovich has refused to consider calls to hold an early election, a year before his term is due to end.
In Kiev, demonstrators on Independence Square held a vigil after dark on Thursday for fallen comrades, lit by mobile phone screens held aloft. Medics carried bodies on stretchers through lines of protesters who chanted "Heroes, heroes" to the dead.
Though armed militants on the barricades tend to be from the far-right fringe, the opposition has broad support. But many Ukrainians also fear violence is slipping out of control.
Kiev residents emptied bank machines of cash and stockpiled groceries, with many staying off the streets.
In further signs of faltering support for Yanukovich, his hand-picked head of Kiev's city administration quit the ruling party in protest at the bloodshed.
In an indication that Yanukovich is also losing support in parliament, the assembly late on Thursday adopted a resolution urging authorities to stop shooting, withdraw police from the centre of Kiev and end the action against the protesters.
But core loyalists were still talking tough. Interior Minister Vitaly Zakharchenko said on Thursday police had been issued with combat weapons and would use them "in accordance with the law" to defend themselves. (Additional reporting by Natalia Zinets, Writing by Timothy Heritage; editing by David Stamp)