Susan E.Rice, Barack Obama’s choice to be ambassador to the United Nations, has criticized the Bush administration for its limited response to the Darfur killings.

CHICAGO - In choosing Susan E.Rice to be ambassador to the United Nations, President-elect Barack Obama has picked an advocate of “dramatic action” against genocide as he rounds out his national security team.

The selection of Ms.Rice, Mr.Obama’s foreign policy adviser, sends a message to the United Nations that the United States will have a prominent and forceful advocate of stronger action, including military force if necessary, to stop mass killings like those in the Darfur region of Sudan in recent years.

To reinforce his intention to work more closely with the United Nations after the tensions of President Bush’s tenure, Mr.Obama plans to restore the ambassador’s post to cabinet rank, as it was under President Bill Clinton, according to Democrats close to the transition.

While the cabinet consists of 15 department heads, a president can give other positions the same rank for the duration of his administration.

“She’s obviously one of Obama’s closest advisers, so it underscores how much of a priority he’s making the position,” said Nancy Soderberg, a senior United States diplomat at the United Nations under Mr.Clinton. “If you look at the last eight years, we obviously need to be more engaged at the U.N. and realistic about what the U.N. can do.”

If confirmed, Ms.Rice at 44 would be the second-youngest ambassador to the United Nations. A Rhodes scholar who earned a doctorate in international relations at Oxford University, she joined Mr.Clinton’s National Security Council staff in 1993 before rising to assistant secretary of state for African affairs at age 32. When Mr.Obama decided to run for president, she signed up as one of his top advisers, much to the consternation of the Clinton camp, which resented what it saw as a defection.

Mr.Obama announced Ms.Rice’s selection at a news conference on December 1, along with his decisions to nominate Senator Hillary Rodham Clinton for secretary of state, keep Robert M.Gates as defense secretary and appoint General James L.Jones, a retired Marine commandant, his national security adviser.

As the ambassador at the United Nations, Ms.Rice will have to coordinate with Mrs.Clinton, but will not be in the White House or at State Department headquarters on a daily basis as major policies are formulated.

Some colleagues from her Clinton and Obama days said Ms.Rice could be blunt and unafraid to “mix it up,” as one put it, on behalf of issues she cares about. Ms.Rice herself acknowledges a certain impatience at times. Admirers said she is a good listener and able to stand up to strong personalities, including foreign autocrats and militants in volatile regions of the world.

“Susan certainly is tough, and she’s tough in exactly the right way,” said Strobe Talbott, a former deputy secretary of state and now president of the Brookings Institution, an independent research and policy institution, where Ms.Rice has worked in recent years. “She’s intellectually tough, she’s tough in her approach to how the policymaking process should work and she will be very effective as a diplomat.”

During her first run at the State Department, Ms.Rice was in charge of responding to Al Qaeda’s 1998 bombing of the United States Embassies in Kenya and Tanzania. But her most searing experience was visiting Rwanda after the 1994 genocide when she was still on the National Security Council staff. As she later described the scene, the hundreds, if not thousands, of decomposing bodies that she saw haunted her and fueled a desire to never let it happen again.

“I swore to myself that if I ever faced such a crisis again, I would come down on the side of dramatic action, going down in flames if that was required,” she told The Atlantic Monthly in 2001. She became a sharp critic of the Bush administration’s handling of the Darfur killings and last year testified before the United States Congress on behalf of an American-led bombing campaign or naval blockade to force a recalcitrant Sudanese government to stop the slaughter.

Jerry Fowler, president of the Save Darfur Coalition, praised the Rice nomination , calling it a powerful sign of the new president’s interest in the issue. The coalition is urging Mr.Obama to begin a “peace surge” of sustained diplomacy to address the continuing problems in Sudan.

“It sends a very strong signal about his approach to the issue of Sudan and Africa in general,”Mr.Fowler said.