The Pakistani Taliban are planning to attack foreigners helping with flood relief efforts in the country, a senior US official has warned.
The Taliban plan "to conduct attacks against foreigners participating in the ongoing flood relief operations in Pakistan", the official told the BBC.
The official also said "federal ministers in Islamabad" may be at risk.
The warning comes as thousands flee their homes in southern coastal areas as floods sweep down from the north.
The UN says more than 17 million people have been affected by the monsoon floods, and about 1.2 million homes have been destroyed.
Some five million Pakistanis have no shelter, and urgently need tents or plastic sheeting to protect them from the sun.
'Plans to attack'
"According to information available to the US government, Tehrik-e Taliban plans to conduct attacks against foreigners participating in the ongoing flood relief operations in Pakistan," the official told the BBC on condition of anonymity.
"Tehrik-e Taliban also may be making plans to attack federal and provincial ministers in Islamabad."
The BBC's James Reynolds, in Washington, says this marks the first time that the US has specifically warned about attacks in connection to the floods in Pakistan.
It is not yet clear what effect this warning will have on US involvement in relief efforts in Pakistan, our correspondent adds.
The US is one of a number of countries to have sent aid and assistance to Pakistan. The US Agency for International Development says that it has so far provided around $150m in support to victims of the flood.
The warning came hours after a top US general involved in the military relief effort said his men had not encountered any security problems in flying aid to Pakistan.
"We have seen no security threat whatsoever in the three weeks we have been operating here," Brigadier General Michael Nagata was quoted by the AFP news agency as saying.
He added that the Pakistani military had done a "highly effective job in providing our force protection and security".
Nations, including the US, have pledged more than $700m (£552m) for relief efforts in Pakistan.
Workers have begun clearing up as the floods recede in the north and the UN has appealed for more helicopters to reach 800,000 people who are cut off.
But some 200,000 people have been evacuated in the Thatta area of the southern province of Sindh, where dozens of villages are submerged.