A court of appeals in Ukraine ruled last week that Tymoshenko should remain in prison on charges she abused her authority in a 2009 natural gas deal with Russia's Gazprom.
State Department spokesman Mark Toner said the U.S. government was disappointed by the court's decision.
"We urge the government of Ukraine to free Mrs. Tymoshenko and the other former government officials currently in detention," he said. "We believe that they should have an unrestricted ability to participate fully in political life, including next year's parliamentary elections."
The decision, he added, was made without respect for the rule of law.
Current Ukraine leaders said the 2009 gas deal caused damage to a Ukrainian economy battered by the global recession. Her Western supporters say the charges are politically motivated.
Her Web site notes the appeals court made the decision in her absence, though it was reported authorities threatened to drag her to court. She's bed-ridden apparently for back complications.
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