WASHINGTON: President Barack Obama's pick for defence secretary is poised to tell Republicans in Congress on Wednesday he might eventually consider reviewing the schedule for withdrawing US troops from Afghanistan, and would push to trim wasteful spending.

Ashton Carter, a former Pentagon No. 2, heads to the Senate for a 9:30 am confirmation hearing that is expected to be far smoother than the one that badly damaged outgoing Defence Secretary Chuck Hagel's political stature two years ago.

But Carter is hardly going to get an easy ride, with key Republicans in the Senate Armed Services Committee sharply critical of Obama's drawdown schedule in Afghanistan as well his limits on US military actions in Iraq, Syria and Ukraine.

“I think he's well qualified. He's going to have to do well. He's going to have to defend the president, but not to a fault,” Senator Lindsey Graham, a key committee member, told Reuters.

Carter was nominated to become Obama's fourth defence secretary after Hagel resigned under pressure last year, raising questions over whether the 60-year-old technocrat would be able break into Obama's tight-knit inner circle. Hagel remains in the job until his successor is confirmed.

In an opening statement Carter is due to read on Wednesday morning, obtained by Reuters, he promised to give Obama his “most candid strategic advice” about the dangers ahead.

Carter also said in written answers to questions due to be publicly released on Wednesday that he's willing to consider recommending changes to Obama's drawdown plans in Afghanistan next year, should conditions deteriorate.

That could appeal to some Republicans who criticise Obama for setting drawdown targets based on what they say is a political calendar instead of security after 13 years of war.

Carter also expressed interest in expanding counter-terrorism cooperation with Pakistan and advancing efforts to arm and integrate Sunni tribal forces into Iraq's battle against Islamic State militants.

Still, it remains to be seen whether Carter will depart from longstanding Obama administration positions.

Carter is an administration insider who has toiled away in high-ranking positions within the Pentagon in recent years.

His roles include deputy defence secretary — the Pentagon's No. 2 job — from 2011 to 2013 and the Pentagon's chief weapons buyer from 2009-11 when he led a major restructuring of the F-35 fighter jet program.

“I would think we will see more of continuity than we will see divergence,” said William Perry, a former US defence secretary and mentor to Carter.

Carter's hearing will be closely watched by Lockheed Martin Corp, Boeing Co and other big weapons makers, which are waiting for clues about Carter's priorities and his commitment to new procurement programs, including a new long-range bomber and the Air Force One replacement.

Like Hagel, Carter criticised across-the-board spending cuts imposed by Congress but also singled out the need for greater reform of defence spending, already underway.

“I cannot suggest support and stability for the defence budget without at the same time frankly noting that not every defence dollar is spent as well as it should be,” he says in prepared opening remarks.