By Sue Pleming

WASHINGTON, April 11 (Reuters) - U.S. senators on Wednesday urged the No. 2 State Department official to hold Libyan leader Muammar Gaddafi accountable for "acts of terrorism" when the U.S. diplomat visits Tripoli in coming days.

Deputy Secretary of State John Negroponte is due to visit Tripoli in the coming week -- the highest-ranking U.S. diplomat to do so in decades -- as part of an African mission from April 11-19 to discuss Sudan's western Darfur region.

"We urge you to use the opportunity your visit presents to send a strong message to Libya's President Gaddafi that he must settle the remaining terrorism cases against his country before he can have fully normalized diplomatic relations with the United States," the seven U.S. senators said in a letter to Negroponte.

The senators, among them Democratic presidential candidate Sen. Hillary Rodham Clinton of New York and Republican Sen. Norm Coleman of Minnesota, were referring to unresolved compensation for U.S. relatives of victims of the 1988 bombing of Pan Am 103 over Lockerbie, Scotland, and a 1986 disco bombing in Berlin, which killed two Americans.

Democratic Sen. Frank Lautenberg of New Jersey said until Libya fulfilled its "legal obligations" to American families affected by terror attacks caused by Libya, Washington should not pursue fully normalized diplomatic ties with Tripoli.

Libya has not agreed to compensate victims of the 1986 Berlin attack. While it reached settlement to pay families of the Lockerbie bombing $10 million per victim, it has not paid out the final $2 million each of the families believe they are still owed.

Asked about the letter, State Department spokesman Sean McCormack said lawyers for the families involved were working closely with the Libyan government.

"We have encouraged a settlement, certainly, but the United States is not a party to those discussions," McCormack told reporters.

State Department officials said Negroponte, who is also visiting Chad and Sudan, was expected to see Gaddafi but an exact time has not yet been set for such a meeting.

The scope of the talks has been billed as dealing with the crisis in Sudan's western Darfur region, but a senior State Department official told Reuters that other issues would also likely be raised.

"John is deputy secretary of state and he will discuss the broad range of issues we have with Libya, but the purpose of his mission is Darfur," said the official, who spoke on condition of anonymity.

The U.S. special envoy to Sudan, Andrew Natsios, also said discussions would be broader than Darfur. "There are other issues as well," he told a Senate committee hearing on Darfur.

Asked whether Negroponte was laying the groundwork for an eventual trip to Libya by U.S. Secretary of State Condoleezza Rice, the senior State Department official said the political context had to be right before the secretary went.

He said Negroponte's trip was not an indication of even closer ties between the two countries but rather that the United States wanted to discuss Darfur and to include the Libyans in trying to resolve the issue.

"They may be doing some things that are unhealthy and could be changed, but there is a lot that they are doing that potentially we could work with them on," said the official.

The United States has been quietly critical of Libya's role in Darfur, accusing Tripoli of trying to set up negotiations with Khartoum that are at odds with the U.N. push for a joint U.N.-African Union peacekeeping force for Darfur.

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