Islamabad: Pakistan summoned the US ambassador on Wednesday to protest over missile strikes by US drones on the Pakistani side of its border with Afghanistan, a Pakistani government spokesman said.

A suspected US drone killed up to 20 militants in the lawless South Waziristan border region two days ago.

"It was underscored to the ambassador that the government of Pakistan strongly condemns the missile attacks which resulted in the loss of precious lives and property," a Foreign Ministry spokesman said in a statement.

"It was emphasised that such attacks were violation of Pakistan's sovereignty and should be stopped immediately."