Two of Pakistan's international airports were put on high alert on Friday because of a terror threat, a security official said.

"We have put Karachi and Lahore international airports on high-security alert after some specific aviation-related threats," Major Riaz Ahmad, a spokesman for the Airport Security Force said.

"Security at both these airport has been beefed up on an intelligence report," he said without providing details.

Earlier this week, the US consulate in Karachi remained closed for three days after its officials received threatening telephone calls. It was not clear whether the threats were linked.

There have been a spate of terror assaults in major Pakistani cities since Islamabad joined the US-led war on terror following the September 11, 2001, attacks in the United States.

Police blame Islamic extremists for targeting Western concerns, religious minorities and senior government officials in an attempt to undermine their country's support for the United States.

Last year, airports in Pakistan were put on high security alert several times.