The United States has canceled the visa of a Ukrainian prosecutor in charge of the case of jailed former premier Yulia Tymoshenko, the latest sign of growing international pressure on Ukraine.

The U.S. Embassy in Kiev said Tuesday that a long-term visa issued to Deputy Prosecutor General Renat Kuzmin has been revoked. The Embassy would not explain its reasons, citing its privacy policy.

Kuzmin protested the decision as illegal and unfair in an unusual a letter to U.S. President Barack Obama earlier this month.

Tymoshenko is serving a seven-year sentence for abuse of office, charges condemned by Western nations as politically motivated. Her conviction seriously strained Ukraine's relations with Washington and Brussels.