CHICAGO — President-elect Barack Obama picked a national security team headed by former campaign rival Hillary Rodham Clinton and Bush administration holdover Robert Gates on Monday, and said he wants to consult with military commanders before settling on a firm timetable to withdraw U.S. combat troops from Iraq.

Obama said a newly completed agreement between Iraq and the Bush administration covering U.S. troop presence signals "a transition period in which our mission is changing" after a long war. The president-elect campaigned on a call to pull out most combat troops within 16 months of taking office, a period he said he still believes "is the right time frame."

Obama named Clinton, a New York senator, as secretary of state and said Gates would remain as defense secretary, a post he has held for the past two years.

At a news conference, the president-elect also introduced retired Marine Gen. James Jones as White House national security adviser, former Justice Department official Eric Holder as attorney general, Arizona Gov. Janet Napolitano as secretary of homeland security and Susan Rice as ambassador to the United Nations.

The announcements rounded out the top tier of the team that will advise the incoming chief executive on foreign and national security issues in an era marked by wars in Iraq and Afghanistan and terrorism around the globe.

"I assembled this team because I am a strong believer in strong personalities and strong opinions," he said.

"I think that's how the best decisions are made. One of the dangers in a White House, based on my reading of history, is that you get wrapped up in group-think and everybody agrees with everything and there's no discussion and there are no dissenting views. So I am going to be welcoming a vigorous debate inside the White House."

"But understand, I will be setting policy as president. I will be responsible for the vision that this team carries out, and I will expect them to implement that vision once decisions are made."

Obama's announcements marked a shift in emphasis after a spate of appointments last week for his economic team, led by Timothy Geithner as treasury secretary.

He now has selected half the members of his Cabinet, and is filling out the top echelons of his administration at an unusually quick pace during his transition as he seeks to fulfill his goal of being able to "hit the ground running" when he takes the oath of office on Jan. 20.

Obama said his appointees "share my pragmatism about the use of power, and my sense of purpose about America's role as a leader in the world."