|President Obama praised General Jones, left, for his dedication as national security adviser [AFP]
General James L Jones, the national security adviser to Barack Obama, the US president, has stepped down from his position.
His place will be taken by Thomas E Donilon, currently the deputy national security adviser.
At a White House ceremony on Friday, Obama praised Jones, a former Marine general, calling him a dedicated public servant who had held one of the most difficult jobs in the administration.
Naming Donilon as his replacement, Obama said: "We have some huge challenges ahead. We remain a country at war."
Donilon, 55, was reportedly against the troop surge in Afghanistan and had counselled Obama last year to resist Pentagon requests for a larger troop increase to combat the Taliban.
Obama eventually agreed to send an extra 30,000 soldiers.
At a Pentagon briefing, Robert Gates, the US defence secretary, tried to play down suggestions of tensions with Donilon, after he was quoted as saying in a new book that Donilon would be a "disaster" as national security adviser.
"I have and have had a very productive and very good working relationship with Tom Donilon, contrary to what you may have read, and I look forward to continue working with him," Gates said.
The resignation of Jones had been expected as part of a mid-term reshuffle at the White House.
Gates has also signalled his intention to resign sometime in 2011. Admiral Mike Mullen, the top US military officer, is also expected to retire.
The series of departures will give Obama an opportunity to reshape his national security team as the United States tries to find an exit strategy from the unpopular war in Afghanistan, to wind down the war in Iraq and to pursue a multi-pronged strategy to end nuclear standoffs with Iran and North Korea.
"Those of us who support the war effort maybe should be a wee bit concerned," Michael O'Hanlon, a senior fellow at the Brookings Institution, said.
The president told congressional leaders in a letter this week he had no plans for any major changes in his Afghanistan war strategy, at least for now. A comprehensive review of that strategy is set for December.
Obama ordered the extra 30,000 troops to Afghanistan last December but also announced they would start coming home in July 2011.
His plan calls for stepped-up training of Afghan forces to take over increasing responsibility from foreign
troops in the war that began in late 2001.