WASHINGTON - The United States on Monday threatened more sanctions on those in Sudan who targeted civilians or blocked a United Nations-led peace efforts and strongly condemned the weekend killings of African Union peacekeepers in Darfur.

State Department spokesman Sean McCormack called for a full investigation of the attack on the Haskanita base in Darfur.

"We are prepared to apply additional sanctions against those who target civilians or peacekeepers and those who block the political process," McCormack said in a statement.

The AU says 20 of its soldiers were killed or injured and nine are still missing after the attack, blamed on rebel factions. It was the worst single assault on AU forces since the 7,000-strong mission was deployed to western Sudan in 2004.

McCormack did not provide details on what kind of sanctions might be imposed or when. But he said they would likely be targeted punitive actions, involving travel bans on individuals deemed belligerent as well as some financial restrictions.

Last May, President George W. Bush tightened U.S. sanctions on Sudan and asked for support for an international arms embargo to end what he calls genocide in Darfur.

International experts estimate 200,000 people have died in Darfur, with 2.5 million driven from their homes as mostly non-Arab rebels took up arms in early 2003 accusing central government of neglect. Khartoum mobilized mainly Arab militias to quell the revolt.

Since then, the fighting has become more complicated, with the splintering of rebel groups into dozens of factions.

McCormack said the United States remained concerned that neither rebel factions nor the government of Sudan were adhering to a cease-fire and urged all parties to stop hostilities immediately.

"The conflict in Darfur will only be solved by a negotiated settlement. We urge all parties to fully participate in the peace process under the leadership of the United Nations and the African Union," said McCormack.

He also called for the rapid deployment of a hybrid U.N./African Union peacekeeping force, which is expected to be 26,000 strong at its peak.