Barack Obama, the US president, is set to nominate former Republican Senator Chuck Hagel as his next defence secretary, a senior administration official has revealed.
Obama, who returned to Washington on Sunday from his Hawaiian vacation, was expected to nominate Hagel as early as Monday.
The nomination would need approval from the Senate, where the former Nebraska senator and Vietnam veteran is likely to face questions over his attitude to Israel, and past calls for military cuts.
Some Republicans already have declared their public opposition to Hagel replacing Pentagon chief Leon Panetta in Obama's second-term Cabinet.
Senator Mitch McConnell, the top Senate Republican, said earlier on Sunday that he was reserving judgment on whether to support his former colleague, Hagel.
But he predicted the former Nebraska senator would face serious questions about his stands on Iran and Israel.
Any nominee must have "a full understanding of our close relationship with out Israeli allies, the Iranian threat, and the importance of having a robust military", McConnell said on ABC television's "This Week".
The former Nebraska Senator has also been attacked by gay rights groups for remarks in 1998 questioning whether an "openly aggressively gay" nominee could be an effective US ambassador. He apologised for the comments last month saying they were "insensitive".
If confirmed, Hagel would become the first former enlisted soldier to lead the Pentagon. He served alongside his brother Tom in Vietnam.
Obama is also expected to round out his security team by unveiling his pick for CIA director to replace David Petraeus, the retired general who stepped down in November over an extramarital affair with his biographer.
Hagel has criticised discussion of a military strike by either the US or Israel against Iran. He also has backed efforts to bring Iran to the table for talks on future peace in Afghanistan.
Some lawmakers have been troubled by his comments and actions on Israel, including his reference to the "Jewish lobby" in the US.
The second-ranking Senate Republican, John Cornyn of Texas, said in a statement that making Hagel defence secretary would be "the worst possible message we could send to our friend Israel and the rest of our allies in the Middle East".
Cornyn did not say he would try to block a Hagel nomination.
Obama, in an interview that aired last Sunday on NBC's "Meet the Press", called Hagel "a patriot" who "has done extraordinary work" in the Senate and on an intelligence advisory board.
Illinois Senator Dick Durbin, the second-ranking Senate Democrat, told CNN's "State of the Union" that Hagel "is a serious candidate if the president chooses to name him".
Republican Sen. Lindsey Graham of South Carolina described Hagel as "a controversial pick".
Obama has nominated Democratic Senator John Kerry of Massachusetts as his next secretary of state, in what was the president's first step in filling out his second-term Cabinet and national security team.
Kerry, as well as the nominees for the Pentagon and CIA, must be confirmed by the Senate.