US defence secretary Leon Panetta has warned senior Pakistani officials that the US is reaching the limits of its patience over the country's failure to root out terrorism.

Stressing that the troops on the Afghan border have the right to defend themselves against the feared Haqqani network, Panetta urged Islamabad to take action against the al-Qaida-linked attacks.

"We are reaching the limits of our patience here," he said during a joint conference with the Afghan defence minister, Abdul Rahim Wardak.

He warned of the possibility that terrorists were finding a safe haven in Pakistan and added: "It is very important that Pakistan take steps to deal with this threat."

The recent increase in lethal drone strikes on suspected insurgents in Pakistan is provoked in part by frustration with Pakistan, a senior US official added.

"Anybody who attacks US soldiers is our enemy. We are not going to take it," he said.

"We have every responsibility to defend ourselves and we are going to make very clear that we are prepared to take them on. We have to put pressure on Pakistan to take them on as well."

The No 2 in al-Qaida, Abu Yahya al-Libi, was killed by a drone strike in Pakistan last week. It was believed to be the 21st US drone strike on the country this year. At least six missiles were fired at a militant compound near Mir Ali near the Afghan border.

Pakistani lawmakers have urged US to stop drone strikes, which have allegedly killed civilians.

But Panetta made clear that the US would defend itself against terrorist organisations.

"And we have done just that. We have gone after their leadership, and we have done it effectively - targeting al-Qaida leadership and terrorists," he said.

"The terrorists who threaten the United States threaten Pakistan as well. This is not just about protecting the United States; it's also about protecting Pakistan. And we have made very clear that we are going to continue to defend ourselves."

The US plabs to withdraw 23,000 American troops from Afghanistan by the end of September, leaving about 68,000 military personnel in the country.