Daniel Inouye, the longest-serving current U.S. senator and a Medal of Honor winner for his heroism in World War II, died Monday of respiratory complications, his office announced. He was 88.

Inouye had represented his state, Hawaii, in the Senate since 1962 and as president pro tempore of the upper chamber of Congress was third in the line of presidential succession after Vice President Joe Biden and Speaker of the House John Boehner.

The Democratic senator was admitted to Walter Reed National Military Medical Center, on the outskirts of Washington, in early December due to respiratory problems.

U.S. President Barack Obama emphasized in a communique that with Inouye's passing the country has lost a "true American hero" and he noted his "incredible bravery" during WWII, during which he was wounded multiple times and lost his right arm due to combat injuries.

Obama, who was born in Hawaii just like Inouye, characterized the senator as a "colleague and mentor."

Meanwhile, Senate Majority Leader Harry Reid, a Democrat, said that he learned of Inouye's death with a "heavy heart," calling him "a giant" in the upper chamber, where he was head of the powerful Appropriations Committee.

Inouye, of Japanese descent, was the highest-ranking Asian-American politician in U.S. history and he was second in longevity in office only to the late Democratic Sen. Robert Byrd of West Virginia.

He was the keynote speaker at the Democratic National Convention in Chicago in 1968 and a member of the Senate committee that investigated the Watergate scandal during the 1970s. EFE