US President Barack Obama has appointed United Nations ambassador Susan Rice to be his new national security advisor, replacing Tom Donilon, who will still attend Obama's two-day meeting in California with Chinese President Xi Jinping that begins on Friday.

In a ceremony held at the White House Rose Garden on Wednesday, Obama said Rice "exemplifies the finest tradition of American diplomacy and leadership", and described her as "passionate and pragmatic".

"Susan is a fierce champion for justice and human dignity, but she is also mindful that we have to exercise our power wisely and deliberately," Obama said, with Donilon, Rice and Samantha Power, a former White House staffer who will replace Rice as UN ambassador, at his side.

Donilon, who will officially step down in July, was praised by Obama as one of the most effective national security advisors in US history, and instrumental in renewing US leadership in the Asia-Pacific region.

"He has worked tirelessly to forge a constructive relationship with China that advances our interest and our values," Obama said.

UN Secretary-General Ban Ki-moon applauded the role that Rice has played as US Permanent Representative to the UN.

"The United Nations as a whole has benefitted from her commitment to strong US engagement with the Organization as the world's principal forum for addressing key global challenges through cooperative and multilateral solutions," he said in a statement.

Orville Schell, director of the Center on US-China Relations at Asia Society in New York, described the 48-year-old Rice as a smart and experienced diplomat, whose close relationship with Obama will help her in her new job.

"It is true that she has not had much experience in working directly on China questions, but she is a quick learner," Schell said.

Schell said he doubted that Rice's limited direct experience in China-related matters would be problematic because Vice-President Joe Biden has a close relationship with Xi. Obama will have the opportunity to get to know Xi more directly and personally at the Sunnylands summit, he added.

"How they are able to interact over the next two days will have a profound effect on the course of US-China relations," Schell said.

Bonnie Glaser, a senior advisor for Asia at the Center for Strategic and International Studies, said Donilon also had little China experience when he took over as national security advisor at the National Security Council, but gained experience on the job.

Glaser said Rice gained a great deal of experience working with China at the UN Security Council on three important issues: DPRK, Syria and Iran.

"Over time, since the president attaches a great deal of importance to the relationship with China and is committed to the rebalance to Asia, Dr. Rice will be even more involved in the US-China relationship," Glaser said.

Zbigniew Brzezinski, former national security advisor under President Jimmy Carter, criticized Rice at a recent seminar as lacking in diplomacy when she publicly called Russia and China "disgusting and shameful" after they vetoed a UN Security Council resolution on Syria last July.

Rice, a Jamaican-American, withdrew from consideration to replace Hillary Clinton as secretary of state after drawing fierce attack from Republicans, who threatened to block Senate confirmation. Rice was at the center of a political firestorm that focused on the Obama administration's early account for the terrorist attack against the US consulate in Benghazi, Libya, last year. The account was later proved incorrect.

Zhu Zhiqun, a professor of political science and international relations at Bucknell University in Pennsylvania, said Rice's withdrawal from consideration for the job of secretary of state means "already her position and stature have been weakened before taking the new job."

"She will likely not be an agenda-setter, but will work closely with (Secretary of State John) Kerry and other members of Obama's foreign policy team," Zhu said.

Analysts interpreted Obama's appointment of Rice, which does not require Senate confirmation, as a defiant act aimed at his Republican opponents. Wednesday's nomination of Power as the new US UN ambassador, however, does require Senate confirmation.

"Obviously I disagree (with Obama's) appointment of Susan Rice as national security advisor, but I'll make every effort to work (with) her on (important) issues," Senator John McCain of Arizona wrote on Twitter.

Zhang Yuwei in New York contributed to this story.