(CNN) – Longtime Hawaii Sen. Daniel Inouye remained hospitalized Monday at a Washington-area military facility, where he is stable but suffering from "respiratory complications," according to his spokesman.
Peter Boylan said the complications stemmed from procedures initiated last week to regulate Inouye's oxygen intake, and that there was "no time table for his release."
"His family and loved ones are overwhelmed by the outpouring of affection and good wishes for his recovery," Boylan said. "We ask that the people of Hawaii and the nation keep the Senator in their thoughts and prayers."
Last week, the Hawaii senator said he would remain at Walter Reed National Military Medical Center "for my own safety and to allow the necessary observation."
Inouye, 88, said he would be back on Capitol Hill as soon as his "doctors allow it."
He was seen on the Senate floor two weeks ago with an oxygen mask before a ratification vote on a United Nations treaty involving disability rights, and received hospital treatment last month for a minor head injury following a fall.
Inouye was elected to his ninth consecutive term in 2010. Behind the late Sen. Robert Byrd of West Virginia, he is the second longest serving senator in the chamber's history.
He has been a senator for all but three of the Hawaii's 53 years as a state. Before that, the decorated World War II veteran was its first House member.
CNN's Ashley Killough and Kevin Liptak contributed to this report.