Sen. Robert Byrd (D-W.Va.) has been admitted to a Washington-area hospital and is in "seriously ill" condition, his office said in a news release Sunday.

The statement said that Byrd, 92, "was admitted to the hospital late last week suffering from what was believed to be heat exhaustion and severe dehydration as a result of the extreme temperatures." The region has experienced a stretch of temperatures in the 90s with high humidity.

Byrd's office said he was not expected to remain in the hospital more than a few days, but "other conditions have developed which has resulted in his condition being described as 'serious.'"

Byrd is the longest-serving member in congressional history, elected to the House in 1952 and the Senate in 1958. He has served in Congress through 12 presidents.

Struggling with frail health in recent years, Byrd was hospitalized three times in 2009.