BY JAMES MARSON

KIEV, Ukraine—A senior U.S. official said Monday that reports of the ill health of jailed former Prime Minister Yulia Tymoshenko are "of great concern," increasing pressure on Ukraine in a case that has already frayed once-close ties.

The U.S. and the European Union have criticized the jailing of the opposition leader on abuse-of-office charges in October as selective justice, and urged President Viktor Yanukovych to find a way ...