Speaking to reporters from his family's estate outside the eastern city of Lahore on Monday, Sharif said the drone strikes pose a "challenge" to Pakistan’s national sovereignty.

"Drones indeed are challenging our sovereignty. Of course we have taken this matter up very seriously. I think this is a very serious issue, and our concern must be understood properly," said Sharif.

Sharif's Pakistan Muslim League-N party appeared on course to secure a majority of seats in Pakistan's parliament and form the next government after claiming victory in Saturday's election.

Pakistan’s tribal regions are often attacked by US assassination drones, with Washington claiming that militants are the targets. However, casualty figures clearly indicate that civilians are the main victims.

The London-based Bureau of Investigative Journalism said in a report released in February that the United States has carried out more than 360 assassination drone strikes in Pakistan since 2004, killing nearly 3,500 people.

The killing of Pakistani civilians, including women and children, has strained relations between Islamabad and Washington.