The U.S. ambassador to Ukraine has met with imprisoned former Prime Minister Yulia Tymoshenko, and renewed criticism of the country over her treatment.

Tymoshenko was sentenced to seven years in prison last year for abuse of office in a case that critics say was political revenge by her arch-foe, President Viktor Yanukovych.

The former prime minister is currently undergoing treatment for a back condition. Ambassador John Tefft met with her in hospital on Monday.

Tefft told reporters after the visit that the U.S. position "has been all along that selective persecution of Ms. Tymoshenko and members of her party is something we disapprove."

He added that the U.S. has called for her release.