Security officials in northwestern Pakistan say a U.S. drone strike has killed at least two suspected militants.

Authorities say the strike took place Saturday in the South Waziristan tribal district, which borders Afghanistan.

Drone strikes have become an increasingly contentious issue between Washington and Islamabad. Relations between the allies plunged into crisis after U.S. air strikes killed 24 Pakistani soldiers in November, resulting in Pakistan shutting its Afghan border to NATO supplies.

Pakistan has demanded the U.S. apologize for the November air strike. Washington has refused.

Pakistan says the missile attacks are counter-productive, violate its sovereignty, kill civilians and fuel anti-U.S. sentiment.