Deputy Secretary of State Robert B. Zoellick arrived in Kenya on Monday on a six-day mission to press for a political resolution to the two-year conflict in Sudan's western Darfur region and also to help solidify a peace agreement in the 21-year civil war between Sudan's north and south.

"My key message is that for this to work, you have to have a political and not a military solution" in Darfur, Zoellick told reporters on his plane to Nairobi. "They've got to regain momentum and make it work. The key goal is to develop a clear message to all the parties that they need to apply themselves to the hard work of peace."

The trip, which will include several days in Sudan, is Zoellick's fourth to the region since April and one of several dozen diplomatic forays by senior U.S. officials since war broke out in Darfur more than two years ago. Tens of thousands of people have died in the conflict, and more than 2 million have been driven from their homes and land, mostly by government-backed militia forces known as Janjaweed.

Despite sustained international pressure, the conflict has not abated. Last week, two Nigerian peacekeeping soldiers and two drivers were attacked and shot dead. Last month, a group of displaced men living in a Darfur refugee camp grew so angry that they took 34 aid workers hostage to draw attention to the arrest of a popular tribal leader.

Some critics in the United States and Africa say the potential U.S. impact in Sudan has been diluted by contradictory agendas and a reluctance to push the Khartoum government too hard on settling the Darfur conflict because it has been a reliable regional ally in the war against terrorism.

The Bush administration has worked closely with the Sudanese government's top national security aide, Saleh Gosh, who was Osama bin Laden's chief minder when the al Qaeda leader lived in Khartoum in the early 1990s. Yet Gosh has also been described by U.S. officials as a key architect of the war in Darfur, which U.S. officials have characterized as genocide.

Moreover, while the United States is now a partner in the north-south peace agreement with Sudan's Islamic government, the Bush administration must also be sensitive to its domestic Christian constituency, which has long lobbied for helping Christians and others in southern Sudan who complain of ostracism and abuse by the government.

"U.S. policy is confused by multiple competing objectives: wooing a counterterrorism ally, constructively engaging a peace partner in the north-south deal, and countering genocidal policies in Darfur," said John Prendergast, a senior adviser for the International Crisis Group, a Brussels-based nonprofit policy organization. "The Bush administration has not yet learned how to walk and chew gum in Sudan."

U.S. officials have denied they are trying to appease Sudanese leaders. Zoellick said Monday that the Sudanese government could not expect the United States to lift sanctions imposed in 1997 unless it makes more effort to end violence in Darfur and acts more quickly to implement the January peace accord with southern Sudan.

In Nairobi, Zoellick will meet with fractious Darfur rebel leaders, whose internal rivalries have hampered peace talks.

"The rebels in Darfur have to have a common negotiating position," Zoellick said. "Obviously, my message is they must unify."

He told reporters that violence has escalated in the last two weeks in Darfur and that the conflict has spilled into Chad, where at least 200,000 displaced Sudanese have fled and where a new rebel group has said it seeks to topple the president because of his alliance with Sudan.

Sudanese children forced to flee their homes in Darfur last year made camp near the town of Nera. More than 2 million people have been displaced and tens of thousands have died despite international pressure on the government to stop backing the Arab Janjaweed militia.