"The United States is troubled" by the Sri Lankan government's decision, State Department spokesman Sean McCormack said.
"Ending the cease-fire agreement will make it more difficult to achieve a lasting, peaceful solution to Sri Lanka's conflict," said a statement by McCormack.
"We call on both the government and the Liberation Tigers of Tamil Eelam to avoid an escalation of hostilities and further civilian casualties.
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According to Washington, both sides "share the responsibility to protect the rights of all of Sri Lanka's people."
Heavy fighting broke out in northern Sri Lanka between government troops and Tamil Tigers just hours after Colombo announced it was pulling out of the ceasefire agreement.
At least six Tiger rebels and a government soldier were killed in the latest clashes along the de facto border of the mini-state run by the Liberation Tigers of Tamil Eelam (LTTE), the Sri Lankan defense ministry said.
The Tigers said they beat back a military offensive i