During his opening statement before his confirmation hearing, President Obama's nominee for secretary of defense, Ashton Carter, said the U.S. military needs a comprehensive strategy to deal with the crises that the country faces. (AP)

Ashton B. Carter, President Obama’s choice to become the next secretary of defense, promised lawmakers Wednesday that he would keep an independent voice and showed a willingness to differ with the White House over its strategy in several global hot spots.

Carter, 60, a physicist who has held several senior posts at the Pentagon dating to Jimmy Carter’s administration, said he was “very much inclined” to provide arms to Ukraine, would be open to reviewing U.S. troop levels in Afghanistan and would be cautious about releasing prisoners from the military prison at Guantanamo Bay, Cuba — in each case potentially putting him at odds with Obama.

“I’ll be entirely straight and upfront with the president and make my advice as cogent and as useful to him in making his decisions as I possibly can,” Carter told the Senate Armed Services Committee in a relatively smooth nomination hearing that seemed likely to lead to his confirmation. “That’s what I’ve pledged to do. That’s what I will do.”

Some of the president’s main foreign policy advisers have long pressed him to take a more aggressive stance in conflict zones. The president has been generally wary of committing U.S. troops or taking steps that could exacerbate instability, but many of the problems have festered and drawn increasing criticism from Congress, including Democrats.

Carter was careful not to directly contradict Obama. Yet he made clear that he would favor providing Ukraine with arms to fend off Russian-backed rebels, something the White House has resisted.

When there’s a presidential nominee, there’s bound to be fireworks. Here’s a look at three of the most testy confirmation showdowns. (Julie Percha/The Washington Post)

“We need to support the Ukrainians in defending themselves,” Carter said in response to pointed questions from Sen. John McCain (R-Ariz.), the committee chairman. “I am inclined in the direction of providing them with arms, including . . . lethal arms.”

The Obama administration has provided the Ukrainian government with night-vision goggles, body armor and other supplies but has drawn the line at funneling weapons out of fear that they would worsen the conflict with the rebels and provoke countermeasures by the Russian government.

Several U.S. military and civilian leaders have pressed the White House to reconsider in recent weeks as the rebels have made gains and Russian President Vladi­mir Putin has shown no sign of halting Moscow’s aggressive intervention in Ukraine.

A senior administration official said Wednesday that the White House was “reevaluating our security assistance” to Ukraine in light of intensified fighting there and that Vice President Biden would discuss the issue with Ukrainian President Petro Poroshenko and German Chancellor Angela Merkel this week in Europe.

“Our goal here is to find a diplomatic resolution to the conflict,” the senior official told reporters, speaking on the condition of anonymity under ground rules set by the White House. “We do not see a military resolution in the offing. And in that respect, our sanctions policy as well as our security assistance are aimed at changing the incentive structure facing Russia and encouraging them to settle this conflict at a negotiating table.”

Russian leaders have had a muted reaction to the renewed push in Washington to arm Ukraine, and there was no immediate response to Carter’s declaration on Wednesday.

Increasing military support for Ukraine could open fissures between the United States and Europe, which so far have been careful to present a unified front on sanctions against Russia.

Merkel said this week that she would not be willing to send German arms to Ukraine and that there was no military solution to the conflict. Other European nations with close ties to Russia may object even more strongly, raising the prospect that they would veto further sanctions against Moscow, which in the European Union require a consensus of all 28 member states.

Inside NATO, there are mixed opinions about the wisdom of increasing military support for Ukraine. Some officials, speaking on background to discuss sensitive planning discussions, say there is no reason to hold back from shipping weapons to Ukraine, given that little else has deterred Russia.

Other officials say weaponry that could change the course of battle would be too sophisticated for the Ukrainian army to operate on its own. Such aid would require Western military forces on the ground to assist — a step that would risk a far broader confrontation with Russia.

Support for arming Ukraine has grown on Capitol Hill. Shortly after Carter’s confirmation hearing, a bipartisan group of 11 members of the Senate Armed Services Committee said they would hold a news conference Thursday to urge the White House to supply weapons to Kiev.

Carter’s warm reception from the panel contrasted sharply with the last time Obama nominated someone to lead the Pentagon. Two years ago, former senator Chuck Hagel (R-Neb.) barely passed muster with the Senate and eventually fell out of favor with the White House as well. He agreed in November to step aside as soon as Obama could find a replacement.

Although Carter faced no significant opposition, lawmakers used the hearing to air a broad critique of Obama’s policies and to press Carter on whether he would favor any changes.

When Carter offered a carefully worded defense of the White House’s strategy for fighting the Islamic State, for example, McCain was dismissive. “Doesn’t sound like a strategy to me,” he said.

Even some Democratic lawmakers questioned the administration’s overall approach to defeating the Islamic State while containing Iranian ambitions in the region. “Is that a coherent strategy?” asked Sen. Jack Reed (R.I.), the senior Democrat on the committee. Carter said it was.

Sen. Kelly Ayotte (R-N.H.) urged Carter to “not succumb to any pressure” from the White House to move quickly to empty Guantanamo. Although Obama has pledged for years to close the prison, many in Congress have questioned his renewed effort to release longtime inmates, saying some could still pose a threat.

“I understand my responsibilities,” replied Carter, who as defense secretary would have to give final approval to any prisoner transfers. “As in everything else I do, I’ll play it absolutely straight.”

On Afghanistan, Carter said he backed Obama’s pledge to withdraw all U.S. troops from the war zone by the end of next year. But in a nod to criticism from some lawmakers that the timetable is too hasty, he said he’d be flexible.

In opening remarks, Carter said he believed that defense spending is a mess and replete with waste — an assessment rarely shared by other officials at the Pentagon.

“I cannot suggest support and stability for the defense budget without at the same time frankly noting that not every defense dollar is spent as well as it should be,” Carter said. “The taxpayer cannot comprehend it, let alone support the defense budget, when they read . . . of cost overruns, lack of accounting and accountability, needless overhead and the like. This must stop.”

“Every company, state and city in the country has had to lean itself out in recent years, and it should be no different for the Pentagon,” he added.

Michael Birnbaum in Moscow contributed to this report.