Al Qaeda was in the late stages of planning an aerial suicide attack against the US consulate in Karachi when Pakistani authorities rolled up a terror cell earlier this week, US sources told Reuters on Friday.

"Operatives were planning to pack a small fixed-wing aircraft or helicopter with explosives and crash it into the consulate," a Department of Homeland Security advisory to pilots and airports to watch for suspicious activity said.

The plot was revealed by one of the six members of an Al Qaeda cell captured in a raid in Karachi on Tuesday in which 150 kg of high explosives were also seized, US sources said.

At least some of the captured group, which included a suspected mastermind of the USS Cole bombing in Yemen in 2000 and a nephew of a senior Al Qaeda leader, Khalid Sheikh Mohammed, had been plotting a strike on the consulate, sources said.

Despite those arrests, US authorities remained vigilant because past suspected Al Qaeda operations involved multiple simultaneous attacks such as the September 11, 2001, hijacked plane strikes on the World Trade Center and Pentagon, and the 1998 bombings of two US embassies in East Africa.

The Department of Homeland Security advisory did not mention any specific threat inside the United States. "We issued this advisory in response to general intelligence regarding threats to airlines, not to a specific threat," a spokeswoman for the agency said.

The advisory was based on information and analysis from the Terrorist Threat Integration Center received during the last 24 hours, it said. The center, which aims to be a hub for terrorism threat information, officially opened on Thursday.

"This plot and a similar plot last year to fly a small explosive-laden aircraft into a US warship in the Persian Gulf demonstrate al Qaeda's continued fixation with using explosive-laden small aircraft in attacks," the advisory said.

A small plane loaded with explosives would be equivalent to a medium-sized truck bomb, it said.

The advisory warned that al Qaeda might attempt to use charter or small aircraft for future attacks because of their "availability, less stringent protective measures, and destructive potential."