Baghdad: Twenty militia fighters were killed in clashes with US forces overnight during the "hottest night" in weeks, after cleric Moqtada Al Sadr threatened to declare open war, a US military spokesman said.

Lieutenant-Colonel Steven Stover described a series of gunbattles and air strikes in the Sadr City slum, stronghold of the cleric's Mahdi Army fighters, in which a total of 20 fighters died.

"I would say it's been the hottest night in a couple of weeks," he said.