"This was not an act of war, it was plain cold murder," Cook said at the Labour Party conference in Blackpool.
The details of the massacre in the village of Obrinje in which 16 people, including young children, where shot in the head, has caused a wave of international revulsion, not least from Labour members at the conference. They are keen to see evidence of the Government's often declared robust policy against Serbian President Slobodan Milosovic.
More evidence emerged yesterday from human rights observers of further brutal attacks against civilians in a different part of the Drenica region of Kosovo. Lying wounded on the ground, a young villager told reporters that he was the sole survivor of a second massacre by Serb troops on Saturday in which 14 of his neighbours were slaughtered.
Moves were also afoot to send war crimes investigators from the International Criminal Tribunal in the Hague to examine the massacre scene.
The UN has already authorised military force against Serbia for refusing to end its repression against ethnic Albanian supporters of the independence movement, the Kosovo Liberation Army.
The Security Council is today expected to request a report from UN Secretary General Kofi Annan on whether Serbia has complied with last week's Security Council resolution calling for an immediate a cease-fire, the withdrawal of Serbian troops, and the start of "serious dialogue on real autonomy" for the province.
Diplomats pointed out that a further report is hardly necessary as a team of diplomats from the six-nation Contact Group were among the first at the massacre scene on Tuesday and have already reported on the carnage.
Britain takes over the Presidency of the council today and Mr Cook indicated that Nato air strikes could follow the receipt of Mr Annan's report early next week. A damning report would it is thought be the trigger for authorising air strikes.
"Military action will flow from a political decision," Mr Cook assured a Blackpool press conference.
Yesterday the outgoing German government threw its weight behind military action announcing that it is making available 14 Tornado fighter bombers.
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