Ukrainian President Viktor Yanukovych has rejected calls to intervene in the trial of former prime minister and political rival Yulia Tymoshenko.

Mr. Yanukovych has been criticized by the West for his government's handling of the Tymoshenko trial, which many Western governments say appears to be politically motivated.

Replying specifically to a letter from Czech President Vaclav Klaus, Mr. Yanukovych said he could not, and would not, interfere in the work of the courts, calling such action “unacceptable.” He added that the Ukrainian court system is independent and unbiased.

Mr. Klaus sought assurance from Mr. Yanukovych that Tymoshenko's trial is not politically motivated.

The charges against Tymoshenko stem from a 2009 deal that ended Russia's cutoff of natural gas to Ukraine. Ukrainian prosecutors say she broke the law by not consulting with her government before signing the deal, which they say was not advantageous for Ukraine and cost the country $190 million. She has denied the charges.

The former prime minister was jailed last week after being held in contempt by the trial judge. The judge has ignored U.S., European and Russian criticism, and has kept her in jail as her trial proceeds.

The gas deal signed by Tymoshenko was among the topics covered in a meeting Thursday in Sochi between Mr. Yanukovych and Russian President Dmitry Medvedev. The Ukrainian leader said the two sides need to seek a compromise in their dispute over gas prices.

Earlier this week, the Czech Republic, Hungary, Poland and Slovakia expressed concern about Tymoshenko's case and the rule of law in Ukraine, saying all Ukrainians are entitled to fair, transparent and independent judicial proceedings. The United States also expressed concern.