WASHINGTON, Sept. 9 - Secretary of State Colin L. Powell declared Thursday that the United States viewed the killings, rapes and destruction of homes in the Darfur region of western Sudan as genocide, and he called on the United Nations Security Council to recognize that the situation required urgent action.

While the declaration has no immediate effect on the role or obligations of the United Nations, said Fred Eckhard, spokesman for the Secretary General, Kofi Annan, it could be viewed as tantamount to invoking Article 8 of the 1948 Convention on the Prevention and Punishment of the Crime of Genocide - the first time that any nation had invoked that provision calling upon the United Nations to take action.

In toughly worded testimony before the Senate Foreign Relations Committee, Mr. Powell said he had concluded that genocide had occurred after studying the findings of experts who had interviewed victims of violence in western Sudan, where attacks have been carried out by government-backed militia known as the Janjaweed.

"When we reviewed the evidence compiled by our team," Mr. Powell said, "we concluded - I concluded - that genocide has been committed in Darfur and that the government of Sudan and the Janjaweed bear responsibility, and that genocide may still be occurring."

After Secretary Powell's testimony, the White House released a written statement from President Bush echoing his determination that the violence in Sudan amounted to genocide. "We urge the international community to work with us to prevent and suppress acts of genocide," Mr. Bush said in the statement.

But even as Mr. Powell delivered the administration's verdict on genocide, there were signs of trouble for a draft Security Council resolution on Sudan threatening penalties if Khartoum did not rein in the militias and permit an outside force to secure Darfur.

Objections were raised by the ambassadors of both Pakistan and China, two countries that have warned that threats of sanctions will backfire and make Sudan more intransigent.

"We want to concentrate on keeping the government of Sudan engaged and not go down a path that could terminate that engagement," said Munir Akram, the Pakistani ambassador. The Chinese ambassador, Wang Guangya, suggested that China might veto such a resolution.

Mr. Powell's call for action was aimed at underscoring a sense of urgency at the United Nations, State Department officials said. But there was also a political dimension to his testimony.

Some critics of the Bush administration, including Senator John Kerry, the Democratic presidential candidate, have called on the United States to take a more assertive role in ending the conflict in Sudan. There has also been criticism from African-American groups, including a group of ministers who support the White House on other matters.

Congress has passed a resolution declaring the Sudan situation genocide, and last week Mr. Kerry called on the administration to follow suit. But until now, Mr. Powell has said that he did not want to use the word without examining the facts, and furthermore said that using the word by itself would not accomplish very much.

Mr. Powell repeated the point on Thursday, but he said the United States and others would continue to put pressure on Sudan by threatening economic sanctions and encouraging a settlement of the rebellion in Darfur that prompted the Janjaweed to retaliate.

Talks to bring about such a solution, including sending at least 3,000 troops led by Nigeria, have been taking place in the Nigerian capital of Abuja.

"Some seem to have been waiting for this determination of genocide to take action," Mr. Powell said. "In fact, however, no new action is dictated by this determination. We have been doing everything we can to get the Sudanese government to act responsibly. So, let us not be too preoccupied with this designation."

A moment later, he added: "Call it civil war. Call it ethnic cleansing. Call it genocide. Call it 'none of the above.' The reality is the same. There are people in Darfur who desperately need the help of the international community."

The Sudan conflict, which erupted last year, has led to tens of thousands of deaths and the displacement of 1.5 million people. The term genocide has long been a sensitive and emotional one in international conflicts.

In 1994, in a situation that some critics say was similar to the current one in Sudan, the Clinton administration at first resisted using the word genocide to describe atrocities in Rwanda. It was only after 800,000 people had been killed that the word was applied. President Clinton later expressed regrets for not acting more quickly.

Secretary Powell, however, has said the administration has been acting even while not using the word. Aides say Mr. Powell waited to use the word, in part because he wanted the State Department report to be completed, and also out of concern that its use would merely antagonize the Sudanese government.

Mr. Powell has said several times that even threats of sanctions can backfire in certain situations, and he has instead tried to meet with Sudan's leaders and apply more low-key pressure. He visited the affected area in June.

"There's an overall reluctance to impose severe sanctions against Sudan at the moment because people are unsure as to whether they would have the desired effect," Mr. Powell told the Senate panel, referring to the Security Council. "So I think we've got a lot of work to do before we could get the kind of sanctions that would actually change behavior of the authorities in Khartoum."

The administration is also concerned that threats and punishments against Sudan would antagonize the Arab world, which already sees the Bush administration as being tooeager to punish it. The conflict in Sudan is waged by an Arab-dominated government against non-Arab people in Darfur.

The decision to use the word genocide on Thursday brought praise from some who have called on the administration to do more.

Richard C. Holbrooke, a former United Nations ambassador in the Clinton administration, said using the word was the correct thing to do. "Should he have done it earlier?" Mr. Holbrooke said in an interview. "Probably. But the important thing is to put maximum pressure on the government in Khartoum, and this is a significant step."

On Thursday evening, the United States ambassador to the United Nations, John C. Danforth, expressed impatience with the resistance to sanctions from China, Pakistan and others. Any country that vetoes a resolution calling for sanctions, he said, "would have to explain the continuing tragedy of Darfur" and explain why it favored "stepping back and letting people die and be shot down by helicopters and raped."

Warren Hoge contributed reporting from the United Nations for this article.