US defense secretary-designate Chuck Hagel cleared a key bar to his Senate confirmation Tuesday, winning the backing of a powerful Democrat who had been concerned about his stances on Iran and Israel.

Senator Chuck Schumer said he met Hagel for 90 minutes on Monday and had been convinced to endorse the blunt former Republican senator to run the Pentagon in Obama's second term which begins on Sunday.

"It was a very constructive session. Senator Hagel could not have been more forthcoming and sincere," said Schumer, an influential figure in Washington known for hawkish support of Israel over the threat posed by Iran.

"Based on several key assurances provided by Senator Hagel, I am currently prepared to vote for his confirmation. I encourage my Senate colleagues who have shared my previous concerns to also support him," Schumer said.

In his statement, the New York senator said that Hagel had cleared up several questions related to past positions and statements, and vowed to do "whatever it takes" to stop Iran getting nuclear weapons.

Hagel, a decorated Vietnam War veteran who has become a skeptic of US land wars abroad, had previously expressed reservations about the wisdom of using military force, leading critics to say Iran would view him as a dove.

"He said his 'top priority' as Secretary of Defense would be the planning of military contingencies related to Iran. He added that he has already received a briefing from the Pentagon on this topic," Schumer said.

Obama is hoping to conclude a deal with the Islamic Republic to end its nuclear program but has warned that he is prepared to use military force if diplomacy fails.

Hagel critics had also hammered him for his failure as a senator to sign a letter calling on the European Union to list Hezbollah as a terror group.

"On Hezbollah, Senator Hagel stressed that -- notwithstanding any letters he refused to sign in the past -- he has always considered the group to be a terrorist organization," Schumer said.

"On Hamas, I asked Senator Hagel about a letter he signed in March 2009 urging President Obama to open direct talks with that group's leaders.

"In response, Senator Hagel assured me that he today believes there should be no negotiations with Hamas, Hezbollah or any other terrorist group until they renounce violence and recognize Israel's right to exist."

Schumer said that Hagel had pledged to work towards the on-time delivery of F-35 joint strike fighters to Israel and concluded that Hagel's views on the threat facing Israel in the Middle East were "geniune."

He also said that Hagel now backed Obama's repeal of the ban on gays serving openly in the military, after past anti-gay comments.

Hagel still faces tough questions in his Senate confirmation process, with former Republican colleagues gearing up to question him over his opposition to the Iraq war surge and positions on the use of US military force.

Cabinet appointees require a simple majority vote in the Senate but Republicans could in theory use their power of filibuster to stall his nomination coming up for a vote.