KABUL—The U.S. and Britain have deployed at least four special-operations teams to the Afghan province of Helmand, stepping up their direct intervention in support of struggling Afghan government forces trying to fight off advances by Taliban militants.

The Taliban have taken control of at least four districts in Helmand, a major focus of U.S. and U.K. combat operations until late last year, and now threaten areas bordering the provincial capital Lashkar Gah, local officials said.

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