The departure of the State Department's No. 3 official adds uncertainty to a U.S. nuclear deal with India that is already in deep trouble.

The United States announced Friday that Undersecretary of State for Political Affairs Nicholas Burns would step down in March. Though Secretary of State Condoleezza Rice said he would remain as the chief U.S. negotiator for the deal even after his departure, it was not immediately clear what his role will be.

It also was unclear whether his resignation as a full-time diplomat would set back dwindling hopes to complete the deal during the Bush administration, which ends in a year.

"It could go either way," said Sharon Squassoni, a nonproliferation expert at the Carnegie Endowment for International Peace, who opposes the deal. "You could see this as a concession that it is looking increasingly unlikely that all the pieces will be in place to get the deal through Congress, and that it is not going any further."

But Squassoni also noted that the new arrangement could help Burns focus on India, which proponents hope is correct. In his current position, Burns is responsible for a host of time-consuming issues, including the negotiations on U.N. sanctions against Iran and on Kosovo's likely declaration of independence from Serbia, which the United States supports.

In a ceremony with Rice to announce his departure, Burns said he would look for opportunities in the private sector after leaving his post.

He said he was leaving because it is "time for me to meet my obligations to my wife and three daughters, and it's time to pursue other ventures outside the government."

Some proponents of the deal argue that the new arrangement will allow Burns to spend more time ensuring that the Indian deal gets swift approval by international regulatory bodies in coming months. Burns, a well-known official with U.S. lawmakers, also will be able to make the case for approval by Congress, where some members have misgivings about it.

"I think he is willing to see this baby through, and I think he will pull it off," said Teresita Schaffer, a former U.S. ambassador to Sri Lanka and current director of the South Asia program at the Center for Strategic and International Studies, who supports the deal. "I don't see this as any administration signal that they are giving up."

State Department spokesman Sean McCormack said the administration has no doubt that Burns is up to the job, noting his reputation for hard work.

"Nick's one of these guys that manages to find 27 hours in the day," McCormack said.

Nevertheless, his task will be formidable.

The deal would allow the United States to send nuclear fuel and technology to India, which has been cut off from international atomic markets because of its refusal to sign nonproliferation accords or accept their inspection regimes and its testing of nuclear weapons.

Although most major opposition in Congress has been countered, the deal still faces tough questions in India. The government has set up a committee to examine the pact, which Indian critics say could cap the country's nuclear weapons program and would allow the United States to dictate Indian foreign policy.

The agreement still faces several other approvals even if the Indian parliament should accept it.

Opponents in the United States say the extra fuel the measure provides could boost India's nuclear bomb stockpile by freeing up its domestic uranium for use in weapons.

That, they say, could spark a nuclear arms race in Asia, where India's neighboring Pakistan and China already are nuclear-armed.

Some of those critics think that more hard work by Burns will not be enough to win a deal.

"If he maintains his focus, I can't see that this announcement will have a great effect," said Daryl Kimball, executive director of the Arms Control Association. "But this is a deal that is already on life support."