The U.S. State Department says the arrest of former Ukranian Prime Minister Yulia Tymoshenko appears to be politically motivated and that she should be released immediately.

The former prime minister is one of about 400 former officials being investigated for crimes committed while in office. Critics say the only cases that have come to trial involve political figures opposed to the government of President Viktor Yanukovych.

Deputy spokesman Mark Toner in a statement Monday urged the Kyiv government to review Ms. Tymoshenko's incarceration, which he says is raising concerns about the rule of law in Ukraine.

Ignoring U.S., European and Russian criticism, a Ukranian judge has kept Ms. Tymoshenko, a leading opposition politician, in jail since Friday as her court trial proceeds.

Toner said Washington has made its concerns known to Ukraine's government and will continue to monitor the proceedings against the former prime minister as well as other Ukrainian opposition figures.

The former businesswoman rose to fame in 2004 as a leader of Ukraine's “Orange Revolution” street campaign, which overthrew Mr. Yanukovych. In 2010, Ms. Tymoshenko lost the presidential race to Mr. Yanukovych.

Ms. Tymoshenko was expected to be the main opposition candidate in next year's presidential elections. If convicted, she will not be eligible to run.

U.S. Senator Richard Lugar said in a statement Monday that Ms. Tymoshenko's arrest appears to be an attempt to settle old political scores.

Ms. Tymoshenko is charged with abuse of power in connection with a 2009 deal that ended Russia's midwinter gas cutoff to Ukraine. Mr. Yanukovych's government says she broke the law by not consulting with her then-government when signing the deal.