Ukraine's former prime minister Yulia Tymoshenko went on trial on Thursday and faces up to 12 more years in prison if convicted of fresh graft charges, in a case that once again drew harsh criticism from an attending Western human rights official.

She refused to attend Thursday's hearing citing poor health.

A key opponent of President Viktor Yanukovich, Tymoshenko was sentenced last October to seven years in prison for abusing her powers as prime minister. The case soured Ukraine's ties with the West which saw it as politically motivated.

In the new trial, she is accused of tax evasion, fraud and attempted theft related to her work as the head of major gas trading company UES, now defunct, in the 1990s.

Tymoshenko has denied any wrongdoing in both cases, dismissing them as part of a campaign of repression against the opposition by Yanukovich's government.

There is no doubt that the charges are politically motivated, Francois Zimeray, the French ambassador for human rights, told reporters after attending the hearing in the city of Kharkiv where Tymoshenko is in prison.

The European Union has warned Ukraine that its members would not ratify key bilateral agreements on political association and free trade while Tymoshenko remains in prison.

But Yanukovich has refused to intervene, indicating he could only pardon Tymoshenko once all trials against her are over.

Ukrainian prosecutors are also investigating Tymoshenko over what they say was her possible involvement in the 1996 contract killing of a parliament deputy, a charge she also denies.

Tymoshenko has appealed her first conviction in the European Court for Human Rights which has not delivered a verdict but has asked Ukraine to ensure she receives proper medical treatment.

Tymoshenko, 51, has been suffering from back pain for months due to a herniated spinal disc, her lawyers and family say, and has trouble walking.

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

After losing power, Tymoshenko and a number of her allies faced corruption-related charges in what she described as a crackdown on opposition and what the European Union said were examples of selective justice.

This is very regrettable because we value highly our relations with Ukraine, Zimeray said. Such cases push the country into isolation.

Separately, U.S. ambassador to Ukraine John Tefft told students of a law school in Kiev on Thursday that observers both in Ukraine and abroad believed its judicial system was not free from political interference.

Moreover, Ukraine's abysmal record before the European Court of Human Rights suggests serious problems, he said, according to a transcript published by the embassy.

(Writing by Olzhas Auyezov; Editing by Louise Ireland)