NAIROBI, Nov 7 (Reuters) - Sudan's government and opposition rebels must jumpstart flagging peace efforts by ending squabbling inside both camps, or spiralling violence in Darfur may unravel a tenuous truce, a top U.S. official said on Monday.

U.S. Deputy Secretary of State Robert Zoellick said Sudan could not expect Washington to lift sanctions imposed in 1997 without more effort to end violence and suffering in Darfur.

He said the U.S. mood on sanctions remained "pretty negative," adding Sudan also had to act more swiftly to implement a January 2005 peace deal for southern Sudan.

Some U.S. lawmakers and rights groups have expressed concern that Washington was being too soft on Khartoum and might consider easing sanctions. An upsurge in fighting in Darfur has drawn global attention to the rising death toll and humanitarian crisis, which Washington has called genocide.

"Violence risks unravelling the very fragile situation in Darfur," Zoellick told reporters on board his airplane to Nairobi, where he will meet with the main Darfur rebel leaders on Tuesday before flying to Sudan.

"You have to regain the (peace) momentum by getting people to respect the ceasefire, come up with a coherent negotiating position and when the negotiations resume on Nov. 20, to get the rebels as well as the government ... to make more progress towards a peace accord," Zoellick said.

He was referring to African Union sponsored peace talks in Abuja. The ceasefire, which dates back to April 2004, has frequently been broken.

Discord among the main Darfur rebel group, the Sudan Liberation Movement (SLM), has been a key reason for a lack of progress in previous rounds of peace talks in Abuja. Some U.S. officials say it may also be a reason for the rising violence in Darfur, as local rebel commanders jockey for position.

Fears of deeper rifts within the SLM rose last week when members elected a new president after the incumbent failed to attend a rebel congress. Zoellick is due to meet with both men.

CARROTS AND STICKS

The deputy secretary said the main carrots for the rebels were the prospect of U.S. and international aid, U.S. pressure on the Khartoum government to cooperate, and backing for the 7,000-strong African Union peacekeeping mission in Darfur.

During his trip, Zoellick will also push the Islamist-led government in Khartoum for more steadfast support for the peacekeepers -- for example by allowing in a long-awaited shipment of 105 armoured personnel carriers -- and seek greater support from the national unity government for the Abuja talks.

The United States is the largest donor of aid to Darfur, but lawmakers earned criticism from some colleagues and rights groups last week for cutting $50 million in pledged support for the peacekeepers.

Zoellick said he also wanted to push the government to rein in "Janjaweed" militiamen in Darfur, who stand accused of a devastating campaign of rapes, killings and burning during the revolt, saying Khartoum had helped create the militia and must now play a role in demobilising the fighters.

For the government, the greatest incentives the United States could provide were development assistance and the role Washington could play to let Khartoum achieve "some international acceptability," Zoellick said.

The deputy secretary called a previously stated target of reaching a Darfur peace deal by year's end "highly ambitious," but hoped the parties could move beyond a declaration of principles by then to prove progress was being made.