By Sue Pleming

WASHINGTON, Oct 26 (Reuters) - The United States on Friday strongly urged Darfur's key rebel groups to attend mediated peace talks in Libya this weekend and voiced impatience with rebels seen as blocking the political process.

State Department spokesman Sean McCormack said the United States was working hard for "maximum attendance" at the peace talks in Libya, which two main Darfur rebel groups have said they will boycott.

"It is an important opportunity to move the process of political reconciliation forward. We are going to do everything we can to underline that fact for all the potential participants and encourage them to go and make a maximum effort for them to come up with some sort of political accommodation," McCormack told reporters.

Key rebels groups -- the Justice and Equality Movement and the Sudan Liberation Army-Unity faction -- said on Friday they would boycott the talks, which are aimed at ending the 4 1/2-year conflict in which international experts say 200,000 people have died.

U.S. special envoy for Sudan, Andrew Natsios, has made phone calls to rebel leaders in recent weeks to urge them to attend the talks, which are being mediated by the United Nations and the African Union.

Natsios will represent the United States at the Libya meetings, which are being held in Sirte.

The United States is becoming increasingly irritated with some rebel groups and has threatened to impose new sanctions on Sudan's government as well as rebel leaders who do not cooperate with peace moves.

A State Department official, who spoke on condition he was not named, said he did not think there was any immediate plan to impose more sanctions, but that it was important to maintain that strong threat.

"They don't have a free ticket or a free ride," he said of the rebels. "We have been putting a lot of pressure on Sudan's government on a number of fronts but we are also watching them."

He said the rebels were seen as one of the main problems in resolving the conflict. "They are so fractured, the rebel groups are not prepared to participate and really don't give a good excuse why they won't go," he said.

Many civil society groups have agreed to attend the Libya talks, and the official said he hoped their attendance would put pressure on the rebels to go.

The United States and others are trying to set up media outlets in the camps for displaced people in Darfur to get information out about the Libya meetings.

"My sense is that once you get talks started, the focus will be there and that it will become more and more difficult for the groups to remain outside," added the State Department official.

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