Hagel: More US troops to remain in Afghanistan next year than initially planned

(CNN) -- Outgoing U.S. Defense Secretary Chuck Hagel arrived in Afghanistan on Saturday for what could be his last trip there as Pentagon chief.

Hagel's unannounced visit will include stops in other countries in the region.

The trip to Kabul follows his announcement last month that he will step down as soon as the Senate confirms his successor. President Barack Obama has said he will nominate Ashton Carter to replace him.

Hagel, who has served as defense secretary since February last year, was forced out by Obama, several sources told CNN.

During remarks announcing his resignation, Obama praised him as an 'exemplary' defense secretary, calling him critical to various national security accomplishments during his tenure.

He said Hagel's resignation was his own choice.

'If there's one thing I know about Chuck, it is that he does not make this or any decision lightly,' Obama said. 'This decision does not come easily to him.'

Behind the scenes, administration officials told CNN there were a series of discussions in recent weeks between Hagel and the President.

Defense Secretary Hagel to step down White House: Secy. Hagel to resign

The two came to realize that 'a different focus was needed and a change is in order,' an official said, adding that it was not the result of a policy difference.

It's unlikely that the confirmation process for Hagel's replacement will occur before next year, when the new, incoming Republican-led Senate convenes.

A critic of the Iraq war, Hagel took over from his predecessor Robert Gates to oversee the draw down from Afghanistan and a heavily cut Pentagon budget.

The former Nebraska senator was the last Republican in Obama's Cabinet. He is a Vietnam combat veteran.

CNN's Barbara Starr contributed to this report.