* Truce follows death of 26 in violence on Tuesday, including 10 police
* U.S. imposes visa bans on 20 senior Ukrainian officials
* Polish, French, German foreign ministers flying to Kiev Thursday
* Obama urges move to unity government, free elections (Adds more Obama comments, paragraphs 3, 21-22)
By Richard Balmforth and Natalia Zinets
KIEV, Feb 20 (Reuters) - Ukrainian President Viktor Yanukovich said he reached agreement with opposition leaders on a "truce" to halt fighting that has killed 26 people, even as the United States stepped up pressure by imposing travel bans on 20 senior Ukrainian officials.
A statement on the presidential website announced an accord 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 the White House would continue to monitor the situation closely to "ensure that actions mirror words."
"My hope is at this point that a truce may hold but ... ultimately the government is responsible for making sure that we shift toward some sort of unity government, even if it's temporary, that allows us to move to fair and free elections so that the will of the Ukrainian people can be rightly expressed without the kinds of chaos we've seen on the streets," Obama told a news conference in Mexico after a North American summit.
A tense standoff between protesters and riot police continued early on Thursday in Kiev, where the foreign ministers of France, Germany and Poland will later meet Yanukovich before returning to Brussels for a meeting of all 28 European Union foreign ministers to decide on targeted sanctions against those deemed responsible for the violence.
Yanukovich, backed by Russia, had denounced the bloodshed in central Kiev, where protesters have been dug in for almost three months since he spurned a trade deal with the European Union in favor of closer Russian ties, as an attempted coup.
His security service said it had launched a nationwide "anti-terrorist operation" after arms and ammunition dumps were looted.
The violence, the worst since Ukraine's independence from the Soviet Union 22 years ago, provoked a chorus of condemnation from the West.
EU ambassadors discussed a series of possible steps including asset freezes and travel bans in talks on Wednesday, even though some diplomats have doubts about the effectiveness of such sanctions.
"The European Union will respond to the deterioration on the ground, including via targeted measures," European Council President Herman Van Rompuy said in a statement, while holding out the possibility of trade and political agreement with Ukraine if it meets goals agreed on with the EU.
Jumping out ahead of its EU allies, Washington late on Wednesday imposed U.S. visa bans on 20 Ukrainian government officials it considered "responsible for ordering human rights abuses related to political oppression," a senior 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.
While declining to name those affected by the bans, which bar them from applying for visas to travel to the United States, the U.S. official said the restrictions were easily reversible if the situation improved.
While EU officials said they were considering targeted sanctions for the "unjustified use of excessive force by the Ukrainian authorities," they noted Yanukovich himself would be excluded from such measures in order to keep channels of dialogue open.
As well as asset freezes and visa bans, ministers will discuss measures to stop riot gear and other equipment being exported to Ukraine and could consider arms restrictions.
The United States, going head to head with Russia in a dispute heavy with echoes of the Cold War, had urged Yanukovich to pull back riot police, call a truce and talk to the opposition. But Washington appears to have little direct leverage in the crisis.
Despite that, the Obama administration has invited the leaders of Georgia and Moldova to visit Washington in the next two weeks, congressional aides said on Wednesday, in what appeared to be an effort to show U.S. support for neighbors of Russia concerned about the crisis in Ukraine.
While attending the North American summit, Obama called on Ukraine's armed forces to stay out of the conflict and warned there would be consequences for those who "step over the line." The Pentagon said Ukrainian military intervention would harm "our defense relationship."
Ukraine, a sprawling country of 46 million with an ailing economy and endemic corruption, is the object of a tug-of-war at a global level between Moscow and the West. But the struggle was played out at close quarters, hand to hand, in fighting through Tuesday night on Kiev's Independence Square, or Maidan.
Russian Foreign Minister Sergei Lavrov blamed the West for encouraging opposition radicals "to act outside of the law."
Obama disagreed when asked at the Mexico news conference if the crises in Ukraine and Syria reflected difficulties between Washington and Moscow, saying: "Our approach in the United States is not to see these as some Cold War chessboard in which we're in competition with Russia.
"Our goal is to make sure that the people of Ukraine are able to make decisions for themselves for the future, that the people of Syria are able to make decisions without having bombs going off and killing women and children," he said.
Fires were still blazing along the barricaded frontline between the protesters and riot police, but there was no immediate sign of a repetition of Tuesday's violence.
Moscow announced on Monday it would resume stalled aid to Kiev, pledging $2 billion just hours before the crackdown began. The money has not yet arrived, and a Ukrainian government source said it had been delayed until Friday "for technical reasons."
Ukraine's hryvnia currency, flirting with its lowest levels since the global financial crisis five years ago, weakened to more than 9 per dollar for the second time this month.
PRESSURE ON THE PRESIDENT
Yanukovich said he had refrained from using force during three months of unrest but was being pressed by "advisers" to take a harder line: "Without any mandate from the people, illegally and in breach of the constitution of Ukraine, these politicians - if I may use that term - have resorted to pogroms, arson and murder to try to seize power," he said.
He declared Thursday a day of mourning for the dead. The state security service said it had opened an investigation into illegal attempts by "individual politicians" to seize power.
One opposition leader, former world champion boxer Vitaly Klitschko, had walked out of talks with Yanukovich over Tuesday night, saying he could not negotiate while blood was being spilled.
Apparently with an eye to possible sanctions that might have consequences for big business interests, three of Ukraine's richest entrepreneurs have stepped up pressure on Yanukovich to hold back from use of force and make every effort to solve the crisis through negotiation with the opposition.
"There are no circumstances which justify the use of force toward the peaceful population," steel and coal magnate Rinat Akhmetov said in a statement issued late on Tuesday.
Akhmetov, who partly bankrolled Yanukovich's election campaign in 2010 and whose wealth is put by Forbes at more than $15 billion, said: "People's deaths and injuries on the side of protesters and the security forces in street battles are an unacceptable price for political mistakes."
Viktor Pinchuk, another steel billionaire well known in the West for his philanthropic activity, said: "A peaceful solution must be found. It is imperative to refrain from the use of force and find a compromise. ... It is time for all sides to take courageous steps toward compromise."
Dmytro Firtash, a gas and chemicals magnate who is part owner of popular TV channel Inter, said in a statement: We, through our joint actions, must end the bloodshed. We are against radical actions by whomever it might be."
(Additional reporting by Matt Robinson, Marcin Goettig and Pavel Polityuk in Kiev, Sylvia Westall in Kuwait, Adrian Croft in Brussels, Christian Lowe in Warsaw, Matt Spetalnick, Lesley Wroughton, Phil Stewart, Patricia Zengerle, Roberta Rampton and Steve Holland in Washington, Mark Felsenthal in Toluca, Mexico; Writing by Richard Balmforth and Matt Spetalnick; editing by Will Waterman, G Crosse and Peter Cooney)