"Sri Lanka stands on the brink," Brown said in a statement. "We have called repeatedly for the violence to cease.

"The humanitarian agencies must be granted access to civilians caught in the crossfire of a dreadful conflict. We are backing UN efforts to secure an orderly end to the conflict. The LTTE must lay down its arms and allow civilians to leave.

"Sri Lanka must understand that there will be consequences for its actions.

A spokesman for Brown's Downing Street office told AFP the British leader would speak to Sri Lankan President Mahinda Rajapakse by phone later this weekend, and added that Brown had already called Rajapakse "several times to make his concerns clear."

Defence officials said the massive offensive against the separatist army would be over "very soon," putting the entire island under government control for the first time in decades.

Government forces have kept up their assault despite international calls for a ceasefire to save the lives of thousands of trapped civilians.