Pakistan says US drone strikes complicate Islamabad's efforts to win the support of Pakistani people [EPA]

At least 13 suspected armed fighters have been killed in less than 24 hours in missile strikes by suspected US drone aircraft in Pakistan's northwest region on the Afghan border, according to local intelligence officials.

At least eight people died in a missile strike in South Waziristan on Tuesday morning, officials told Al Jazeera. Five people were also killed in a missile strike in neighbouring North Waziristan on Monday.

But the Reuters news agency put the death toll at 30 in at least three separate suspected drone strikes since Monday.

A local intelligence official told Reuters that up to 19 people had been killed when missiles struck a compound. Al Jazeera has not confirmed that incident.

The United States has stepped up missile attacks by remotely-piloted drones on suspected Taliban and al Qaeda-linked fighters on the Pakistani side of the border since the killing of al Qaeda chief Osama bin Laden in Pakistan by US forces in May.

More than 90 fighters have been killed since June in drone attacks, according to Reuters figures based on statements from local intelligence officials.

Pakistan has regularly complained about the US drone strikes, saying they complicate Islamabad's efforts to win the support of the Pakistani people and isolate the insurgents in border regions.

Pakistan-US relations have steadily declined since 2010, with ties severely weakened following the killing of two Pakistanis by a CIA contractor in January, and the US raid that killed bin Laden in May, which Pakistan says was a breach of its sovereignty.

The United States this week said it was holding off $800 million in military aid to Pakistan in a show of displeasure over Pakistan's cutback of US military trainers, limits on visa for US personnel and other bilateral irritants.