"We call upon the government and military of Sri Lanka and the Tamil Tigers to immediately stop hostilities until the more than 140,000 civilians in the conflict are safely out," State Department spokesman Robert Wood said.
"The Sri Lankan government, as legitimate sovereign power, has before it an opportunity to put an end to this lengthy conflict," Wood told reporters.
But he said a lasting peace would only come "through a political solution that addresses the legitimate aspirations of all Sri Lankan communities.
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Sri Lanka said Thursday it had made fresh advances into an area still held by the Tiger rebels, who launched a campaign in 1972 to create a separate homeland on the island for the Tamil minority.
Civilians -- estimated at more than 140,000 by the United States and more than 100,000 by the United Nations -- are holed up in the narrow strip on the northeastern coast initially designated a safe zone.
Arguing the civilians were being used by the rebels as a human shi