Despite the Bush administration's sustained involvement in Sudan, some members of Congress and rights groups worry that Washington is being too soft on Khartoum's Islamist leadership. And an upsurge in violence in Darfur is drawing world attention to a rising death toll and the suffering of refugees.

Tens of thousands of people have been killed and more than two million forced from their homes in fighting since non-Arab rebels took up arms in early 2003 accusing the Arab-dominated central government of monopolizing wealth and power and marginalizing Darfur.

``Helping to end violence, ease suffering and forge a lasting political solution for Sudan's conflicts remain high priorities for the United States,'' U.S. Deputy Secretary of State Robert Zoellick said a few days before his trip. ``The situation remains tenuous.''

This will be Zoellick's fourth trip to Africa's largest country since April.

Eleven months after the United States helped forge a landmark peace deal to end a protracted war in southern Sudan, Zoellick will also push the new national unity government to implement vital elements of the north-south accord, including demobilizing troops and defining internal borders.

The United States says successful implementation of the southern Sudan accord, or comprehensive peace agreement, is essential to convince the Darfur rebels that a peace pact will also bring them a political voice and economic support.

``Trying to bring some unity among the SLM (Sudan Liberation Movement rebels of Darfur) is essential, but also hammering the government to implement the comprehensive peace agreement to give the rebels the confidence is essential,'' said Jendayi Frazer, assistant secretary of State for African affairs.

PRESSURE FROM CRITICS

But critics including several vocal Republican and Democratic members of Congress say Washington must exert greater pressure on Sudan to make peace with rebels and stop support for ``Janjaweed'' militiamen in Darfur accused of rapes, killings and burning in non-Arab villages during the revolt.

Critics say the Bush administration must also push the Khartoum government to fully implement the southern accord.

After lawmakers recently cut $50 million in support for African Union peacekeepers in Darfur, several Republican and Democratic lawmakers called on President George W. Bush to provide more funds to the African Union troops, who are struggling to fulfill their mission.

``Darfur, as you recently reconfirmed, is the scene of ongoing genocide,'' Democratic Sens. Richard Durbin and Jon Corzine, and Republican Sens. Mike DeWine and Sam Brownback, wrote in a letter to Bush.

``The African Union is the primary line of defense against this continuing violence, disruption and terror,'' they wrote.

Zoellick will start his trip in Kenya, where he will urge SLM rebel leaders to develop a unified position before the next peace talks with the Khartoum government in Abuja this month.

Fears of deeper rifts and infighting within Darfur's main rebel group rose on Thursday when members elected a new president after the incumbent leader failed to attend a rebel congress. Zoellick will meet with both men.

Zoellick will also meet with members of Sudan's new unity government and top officials of the AU peacekeeping force.