The United States pressed Ukraine on Tuesday to free jailed ex-premier Yulia Tymoshenko as she entered the 12th day of a hunger strike that may prompt an EU boycott of its Euro 2012 football games.

US Secretary of State Hillary Clinton's statement pushed Ukraine into still further diplomatic isolation just weeks before it begins co-hosting with Poland a tournament watched by billions across the world.

European football authorities conceded they were facing growing calls to move or postpone the once-in-four-year event when it kicks off in co-host nation Poland on June 8 before concluding with the final in Kiev on July 1.

But they said a venue switch at this late stage was impossible despite fears of a humiliating no-show by European leaders to any of the matches played in the four Ukrainian host cities.

Ukraine for its part held firm and accused the West of unfairly using the "language of ultimatums" against its leaders and football fans.

Clinton said Washington was "deeply concerned by the treatment" of the 2004 Orange Revolution leader and pressed for the release of her and "other members of her former government" jailed since they lost 2010 elections.

Yet Clinton also expanded her earlier demands by additionally calling for "the restoration of their civil and political rights" -- a move that could turn Tymoshenko into a future election threat to President Viktor Yanukovych.

Tymoshenko was jailed for seven years in October in a highly controversial case that immediately damaged Ukraine's relations with the European Union and saw her rival Yanukovych accused of waging a personal vendetta.

The authorities have since opened still new hearings against the charismatic but divisive leader relating to tax charges dating back to the 1990s that could extend her stay in prison until 2023.

The 51-year-old launched a hunger strike on April 20 to protest an alleged beating she received by three prison guards. Her supporters later released pictures of stomach bruises on Tymoshenko they said backed those claims.

European concern intensified Monday when the office of European Commission chief Jose Manuel Barroso said he had "no intention" of travelling to Ukraine following a similar decision by EU Justice Commissioner Viviane Reding.

At least five European presidents simultaneously declined invitations to attend a May 11-12 summit in the Black Sea resort of Yalta.

Reports meanwhile emerged of German Chancellor Angela Merkel preparing to instruct her ministers to stay away from the three matches their national team plays in the group stages in Kharkiv and Lviv.

The chancellor of Austria has also confirmed his boycott of matches, and the Austrian defence minister, Norbert Darabos, who also holds the sports portfolio, decided to boycott a June 1 Austria-Ukraine friendly.

The ex-Soviet nation's EU envoy Konstantyn Yeliseyev said Ukraine still held out hope that "reports about calls being made for a Euro 2012 boycott turn out to be an exaggeration".

"This does not promote the constructive development of relations and only turns political dialogue into a language of ultimatums," he told the Interfax-Ukraine news agency in Brussels.

Yanukovych himself has stayed silent on the boycott and has so far issued no public comments on Tymoshenko's hunger strike.

He released a brief May Day message on Tuesday saying the holidays should "reinforce our strength and trust in ourselves".

But Tymoshenko's daughter -- an alumna of the London School of Economics who sat grieving at her mother's side the day she was convicted in Kiev -- urged Ukrainians to rise up in protest and match the concern expressed by EU states.

"If the regime of Yanukovych did this to a former prime minister, imagine what it can do to each one of you," the 32-year-old Yevgenia pleaded in a message posted on her mother's website.