As he left his helicopter, the refugees ringed the square, leaning forward as they gawked. Mr. Zoellick, the deputy secretary of state, waved and smiled. On his left wrist that day, he had chosen to wear a blue jelly plastic bracelet bearing the slogan, "Not on our watch." But even with all his attention on Sudan, the population of this camp, and of others across Darfur, grows every day.

Asked in Khartoum on Wednesday about America's intentions toward Sudan, Mr. Zoellick bristled a bit when he noted that this was his fourth visit in the past seven months. "I haven't been to any other country four times this year," he said. "I haven't even been to New York four times this year."

Mr. Zoellick has thrown the full power and prestige of his office into his mission in Sudan: to end the violence in Darfur that has claimed more than 200,000 lives and to consummate the peace agreement between southern rebels and the government in Khartoum.

But by his account, the results have been disappointing. By almost any measure the situation has worsened since he became involved. Perhaps, he said, Sudan is impervious to outside pressure and entreaty.

Mr. Zoellick had been in the deputy secretary's job only a couple of months when he took on the Sudan assignment and made his first trip here in April. Since then, Sudan has taught him the limits of diplomacy.

"If people are determined to kill each other," he said with a grim smile, "there's not a lot the United States can do."

If nothing else, Mr. Zoellick has grown to understand the Sudanese better. He can recite tribal names, affiliations and political leanings at will and recount the nation's blood-spattered history as if it were his own. So he knows that Western political leaders have seldom fared well here. The grim trend started with Gen. Charles George Gordon, the British administrator of Khartoum who in 1885 was beheaded on the very palace steps Mr. Zoellick climbed Wednesday afternoon, on his way to the office of Sudan's current president, Omar Hassan al-Bashir.

As Mr. Zoellick and many other world leaders have repeatedly done in the past two years, he urged Mr. Bashir to end the violence in Darfur. And as Mr. Bashir has repeatedly done, he told his Western visitors what they wanted to hear, then appeared to go about his business undeterred. On Thursday, government-backed militiamen on horseback galloped into the Kalma refugee camp just hours before Mr. Zoellick arrived. They shot dead one refugee, then rode out again.

Violence in Darfur has worsened, after a relatively calm summer. Darfur's rebel leaders have split in a bitter feud, seriously complicating sputtering peace negotiations. And after the death in July of John Garang, the charismatic southern leader, movement toward carrying out the terms of the north-south peace treaty has slowed.

About all Mr. Zoellick and the Sudanese can cite as significant progress are the recent announcements that Sudan intends soon to appoint members to several commissions that will oversee the details of the north-south peace treaty.

"I didn't start out starry-eyed about this," he said in an interview. "Yes, this causes some frustration." But, he added, Sudan is a harsh place, where citizens live by "a culture of retribution."

Officials in Washington, including 109 members of Congress who wrote to the State Department last month accusing Mr. Bush of appeasing Sudan, want to find a simple villain in the Sudanese government, he said. "But there are no angels here."

The Darfur rebels hold much of the responsibility for the surge in violence, though American officials say the government's typical response is brutal and indiscriminate, leading to the deaths of hundreds of civilians not involved in the conflict.

Asked last week why Sudan should listen to what Americans say, Jendayi Frazer, the assistant secretary of state for African affairs, said, "Because it affects their relations with the United States."

But after months of high-level attention by American officials and only token responses from the government in Khartoum, the administration must be wondering whether Sudan really cares.

Mr. Zoellick insisted he was undeterred. "I'm persistent," he said. But when asked if he held any optimism, he thought for a minute, then quoted a man he respects: J. B. Kazura, the Rwandan general who is acting commander of the African Union peacekeeping forces in Sudan.

The road ahead is "very difficult," the general told him. "Very difficult."