At least seven persons were arrested in Karachi on Thursday in connection with a terrorist plot to blow up commercial airlines during flights from the United Kingdom to the United States.

"Arrests have been made after exchange of information between the two governments," Foreign Office Spokeswoman Tasneem Aslam said from Islamabad without elaborating where and how many arrests had been made.

Sources, however, said seven persons were arrested in Karachi.

Aslam said Pakistan intelligence agencies had tipped off their British counterparts that resulted in averting the disaster.

"The bombing plot was foiled after exchange of information between the two countries," she said.

Meanwhile, Pakistan International Airlines declared high alert on all its international flights. "Passengers will be allowed to board international flights only with transparent hand luggage," A PIA spokesman said.

British Police has arrested 21 people in London [Images], Birmingham and Thames Valley overnight in an ongoing operation. Those arrested are believed to be British citizens with many of Pakistani origin.