Britain's foreign minister and the UN chief said on Monday they were appalled by reports that hundreds of Sri Lankan civilians were killed over the weekend in what the United Nations called a "bloodbath."
Hundreds of people trapped in a war zone were reported killed on Sunday and Monday in artillery barrages that struck the narrow strip of territory controlled by separatist rebels.
A UN spokesman in Sri Lanka said more than 100 children were killed in the "bloodbath" as the government tries to wipe out the last remnants of Tamil Tiger rebels.
British Foreign Secretary David Miliband also raised doubts about whether Colombo could be trusted to use a $US1.9 billion loan from the International Monetary Fund appropriately.
"Our message is a simple one, which is that the killing must stop," Mr Miliband said. "The civilians ... trapped in the zone, up to 50,000 in an area of just 3 square kilometers are the victims of what at the moment is a war without witness."
UN Secretary-General Ban Ki-moon issued a statement through his spokeswoman saying that he, too, was appalled.