US President Barack Obama has accepted the resignation of Defence Secretary Chuck Hagel, who he said requested last month to leave Pentagon's top post.

According to US media, Hagel was pressured by Obama to quit, as Pentagon embarks on a new strategy to deal with the Islamic State in Iraq and Levant (ISIL).

Announcing the resignation on Monday at the White House, Obama, who was flanked by Hagel and Vice President Joe Biden, praised the outgoing defence secretary, describing him as "exemplary".

"When I asked Chuck to serve as secretary of defence we were entering a significant period of transition: the drawdown in Afghanistan, the need to prepare our forces for future missions and tough fiscal choices to keep our military strong and ready," Obama said.

Hagel, who was the sole Republican on the US national security team, struggled to improve his ties with Congress after a contentious 2013 confirmation hearing. He submitted his resignation letter after lengthy discussions with Obama that began in October, officials said.

"It's been the greatest privilege of my life to lead, and - most important - to serve with the men and women of the Defence Department and support their families. I am immensely proud of what we have accomplished during this time," Hagel said.

He will continue in the job until his successor is appointed by the US Senate.

Al Jazeera's Patty Culhane, reporting from Washington, said the move suggests Obama is paying heed to criticism over the handling of ISIL threat in the Middle East.

"Officials are saying that the Pentagon needs an approach different from the one Hagel would bring" if he had stayed, our correspondent said, adding that the Pentagon chief was seen as "timid and a bit confused".

Syria questions

Hagel had raised questions about Obama's strategy towards Syria in a two-page internal policy memo he wrote that leaked. In it, he warned that Obama's policy was in jeopardy due to its failure to clarify its intentions towards Syrian President Bashar al-Assad.

Obama has insisted that the United States can go after ISIL fighters without addressing Assad, who the United States would like to leave power.

Officials said Obama wanted fresh leadership during the final two years of his administration.

"What I can tell you is there are no policy differences in the background of this decision," a senior US defence official cited by Reuters said.

"The secretary is not resigning in protest and he's not being 'fired'," the official said.

Top potential candidates to replace Hagel include Michele Flournoy, a former under secretary of defence, and Ashton Carter, a former deputy secretary of defence, who were rumoured to be contenders for Hagel's job before he was named.

Senator Jack Reed, Democrat of Rhode Island, is another possible contender.  

Source: Al Jazeera and agencies