|Panetta, right, has pushed for a strategy that would bring US combat forces out of Iraq [AFP]
A day after stepping down as CIA director, Leon Panetta has been sworn in as US defence secretary.
Panetta succeeds Robert Gates, who was a Republican holdover from the previous Bush administration, and is the first Democrat to take charge of the Pentagon since William Perry finished his tenure in 1997.
The US Senate approved his nomination last week in a 100-0 vote.
"Republicans and Democrats alike have always found Panetta very user-friendly," said Gordon Adams, who worked for Panetta when he was the White House budget chief in 1993-94.
Lindsey Graham, a Republican senator, called Panetta a "home-run choice" by Barack Obama, the US president, to succeed Gates.
Panetta began settling into his job on Friday by telling members of the military and their families that they are "at the top of my agenda".
In an opening message to the defence department, he acknowledged the fiscal challenges he is inheriting, including an administration request to save $400bn from the defence budge over the next 12 years.
"While tough budget choices will need to be made, I do not believe in the false choice between fiscal discipline and a strong national defense. We will all work together to achieve both," Panetta said in the message distributed by the Pentagon.
Adams said: "The president has a huge budget crisis going right now, and so literally on Friday when Leon steps into the job he's going to find himself in the middle of negotiations about budgets, and it's going to include defence.''
In Panetta's written message, issued moments after his swearing in, Panetta said that in his two and a half years as CIA chief he appreciated the military's capabilities and promised that as Pentagon chief he would do all he could to maintain that strength.
"Our nation is at war," he wrote. "We must prevail against our enemies. We will persist in our efforts to disrupt, dismantle and ultimately defeat al-Qaeda."
He mentioned that his arrival at the Pentagon coincides with the start of a withdrawal of US troops in Afghanistan on a schedule announced by Obama last week.
In 2006, Panetta served alongside Gates on the Iraq Study Group, where he revealed himself to be a skeptic of the war.
He pushed for a strategy that would bring US combat forces out of Iraq and redirect the military's efforts to
focus on al-Qaeda and training Iraqi security forces.
He later called the war "divisive, unstable and dangerous".
Later this year, Panetta may face an Iraqi government request that some of the roughly 47,000 US forces still in the country stay beyond the end of this year, when all US troops are supposed to go home.
Panetta began his public life as a Republican serving in the Nixon administration as a special assistant to the secretary of Health, Education and Welfare and as director of the US Office of Civil Rights.
He switched to the Democratic Party in the 1970s and was first elected to the House in 1976.