BAGHDAD, April 20 (Reuters) - Twenty militia fighters were killed in clashes with U.S. forces overnight during the "hottest night" in weeks, after cleric Moqtada al-Sadr threatened to declare open war, a U.S. 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 Mehdi 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.

(Reporting by Peter Graff; editing by Noah Barkin)

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