Pakistan on Wednesday condemned a "unilateral" U.S. military drone strike inside its territory that security officials said killed two members of the Afghan Taliban-allied Haqqani network, adding to already tense relations between the uneasy allies.

The unmanned aircraft's missiles "targeted an Afghan refugee" camp in Pakistan's northwestern Kurram region, a statement from the Pakistani foreign office said.

"Such unilateral actions, as that of today, are detrimental to the spirit of cooperation between the two countries in the fight against terrorism," the Pakistani statement said.

The U.S.-led military mission in Afghanistan, which trains and assists Afghan government forces fighting the Taliban, said it "regularly conducts offensive operations within the borders of Afghanistan with the intent of bringing security and stability not only to Afghanistan, but also the entire region", but said it had no information on the drone strike reports.

Two missiles hit a house and killed Haqqani network militants Ahsan Khorai and Nasir Mehmood in the village of Dapa Mamozai in Kurram, according to a police officer and an intelligence official.

"Two militants from Haqqani network were killed in the drone attack," the intelligence official based in the area said.

There has been an uptick in U.S. drone strikes inside Pakistan in the mountainous border regions bordering Afghanistan since U.S. President Donald Trump took office in January 2017, though they are a long way off their peak in 2010.

Earlier this month, one man was severely wounded in a suspected drone attack in FATA, while another Haqqani militant was killed on Dec. 26 in a suspected U.S. drone strike inside Pakistan.

Trump has taken a hardline stance on Pakistan, which he says provides safe haven to high-level commanders from the Haqqani network, a group which often conducts deadly attacks in Afghanistan.

Relations between Washington and Islamabad have frayed over the past month in the wake of Trump's angry tweet on Jan. 1 about Pakistan's "lies and deceit" over its alleged support for the Afghan Taliban and their allies. The United States this month also suspended military assistance worth about $2 billion.

Islamabad denies sheltering militants and accuses Washington of not respecting Pakistan's vast sacrifices in the war on militancy.

Over the past decade, almost all U.S. drone strikes inside Pakistan have taken place within the border areas, but some Pakistani officials fear the United States under Trump will begin carrying out strikes outside the tribal areas.

Relations between Pakistan and the United States have endured strain during the 16-year war in Afghanistan, especially after al Qaeda chief Osama bin Laden was killed by U.S. special forces inside Pakistan in 2011.