Ukraine's top human rights official called for an investigation on Wednesday into allegations that Yulia Tymoshenko was beaten by prison guards, saying the former prime minister's treatment could amount to torture.

Tymoshenko's allegations that she was punched in the stomach and dragged from her bed were denied by prison authorities. But the accusation caused further concern in Western capitals over a case that has strained relations between Ukraine and the European Union.

The 51-year-old has been on a hunger strike since Friday, when she said she was assaulted during a forced move to a hospital, and prison authorities warned her on Wednesday that she could be force-fed.

Tymoshenko, the main political rival of President Viktor Yanukovich, was jailed last October. She is serving a seven-year-prison term in the city of Kharkiv following an abuse-of-office conviction described by many in Europe as politically driven.

Nina Karpachyova, parliament's human rights representative, said a member of her staff had visited Tymoshenko and confirmed she was bruised.

The Ombudsman states that escorting Yulia Tymoshenko (to hospital) in such a way amounts to cruel treatment of an inmate and can be regarded as torture, she said in a statement.

The Ombudsman demands that the prosecutor general launch a criminal case and suspend everyone involved in the cruel treatment of Tymoshenko.

The state prison service said on Wednesday that Tymoshenko had been informed that the procedure of compulsory feeding can be applied to inmates who refuse food.

The EU commissioner in charge of enlargement, Stefan Fule, wrote on Twitter that the case was a visible and painful stain on Ukraine.

The European Union has previously said its members would not sign deals on political association and free trade with Ukraine while she remains in jail.

Ukraine's Foreign Ministry said it was trying to arrange treatment by doctors from the German clinic Charite, who have previously visited her, the Interfax news agency reported. Germany has offered to treat Tymoshenko, an offer Foreign Minister Guido Westerwelle said he had again made last week to the Ukrainian government.

Germany's offer to treat her here stands, German government spokesman Steffen Seibert told reporters in Berlin, saying the guards' behaviour, if confirmed, would be unacceptable and completely disproportionate.

Given her poor health you can imagine that a hunger strike would have a major impact. This is why it is more important than ever to allow her to have appropriate medical treatment.

It is not clear if she would agree to be transferred.

Such a move would remove a cause of possible embarrassment for Ukraine's government when thousands of people visit the country this summer for the Euro 2012 football tournament, which uses the stadium in Kharkiv as one of its venues.

Tymoshenko has been suffering from back pain for months, her lawyers and family say, and has trouble walking.

She was leader of the 2004 Orange Revolution which doomed Yanukovich's first bid for the presidency. She has since served twice as prime minister but lost the 2010 presidential vote to Yanukovich in a close run-off.

(Reporting by Olzhas Auyezov; Additional reporting by Alexandra Hudson in Berlin; Editing by Alessandra Rizzo)