The Vietnam war veteran tapped to lead the Pentagon, Chuck Hagel, came under a withering attack from Republican critics on Thursday as he struggled to explain his stance on Israel and Iran.

Hagel endured hours of relentless grilling at his confirmation hearing, with Republican senators citing his blunt comments over the years to portray him as naive on national security and disloyal to Washington's staunch ally, Israel.

The former Nebraska senator chose not to push back when confronted with his often sharply-worded remarks on Israel, saying he regretted referring to the "Jewish lobby" and its influence on Congress.

"I've already said I regret referencing the Jewish lobby," Hagel told the Senate Armed Services Committee. "I should have said pro-Israel lobby."

Hagel stumbled at times and mistakenly said he favored "containment" of Iran instead of President Barack Obama's policy of preventing Tehran from obtaining nuclear weapons. He corrected himself later after being handed a note.

The Republican politician apparently decided to absorb the blows instead of attempting to parry the attacks, perhaps because the White House is confident that the Democratic Party's majority in the Senate will secure his nomination, despite the rough reception at the hearing.

Even if Republican jabs fail to derail his nomination, the campaign against Hagel -- backed up with television attack ads -- left him bruised and served as a warning that he will be under tough scrutiny at the Pentagon.

A White House source said the administration did not sense Hagel's nomination was in any danger, and expressed confidence that all the Democrats on the committee would still vote to confirm him.

The source also said that the Hagel camp had expected tough questions, but was surprised that Republicans dwelt on "trivial" issues like Hagel's response to a caller on an Al Jazeera show, and that there was little discussion on Afghanistan, with 66,000 US troops still on the ground there.

"They spent more time talking about a war that's over than one that's ongoing," the source said.

The most dramatic exchange came when a fellow Vietnam War veteran, Senator John McCain, blasted Hagel for his opposition to the 2007 troop surge in Iraq and demanded Hagel declare if he had been wrong.

But Hagel calmly refused and tried to explain his thinking at the time, even as McCain repeatedly interrupted him.

"I want to know if you are right or wrong. That's a direct question. I expect a direct answer," McCain said.

Hagel replied: "I would defer to the judgment of history."

McCain then responded with disgust: "History has already made a judgment on the surge sir, and you're on the wrong side of it."

McCain sees the troop surge as allowing for a dignified exit from Iraq. Hagel, however, said it was unclear if it was worth losing some 1,200 Americans in the surge of reinforcements to Iraq.

The tense back-and-forth underscored the tensions between Hagel and his fellow Republicans over his apostasy on the Iraq war, which he initially supported before breaking ranks.

Another senator wounded in Vietnam, John Kerry, has been confirmed as the next secretary of state, and both Hagel and Kerry reflect Obama's preference for restraint when it comes to employing US military might.

Decorated with two Purple Hearts for his combat tour in Vietnam, Hagel has said military action should be a last resort and has sometimes expressed impatience with Israel and opposed some unilateral sanctions on Iran.

But Hagel sought to reassure lawmakers, saying he was ready to back military action if necessary against Iran or other adversaries.

"I am fully committed to the president's goal of preventing Iran from obtaining a nuclear weapon, and -- as I've said in the past many times -- all options must be on the table to achieve that goal," he said.

Hagel has pursued a charm offensive in recent weeks, holding a flurry of meetings with senators in Congress. But the ranking Republican on the Senate Armed Services Committee said he was not convinced, portraying Hagel as ready to appease Tehran.

"His record demonstrates what I view as a lack of steadfast opposition to policies that diminish US power and influence throughout the world," Senator Jim Inhofe said.