Attacks that killed 172 people in Mumbai in November could happen in the United States, FBI Director Robert Mueller warned in a speech here.

"How many other cities around the world could fall prey to such an attack? How many cities here, in the United States?" Mueller asked today in a presentation to the Council on Foreign Relations in Washington.

Mueller, who took over the Federal Bureau of Investigation just days before September 11 2001, said the Mumbai attacks showed that terrorists "with large agendas and little money can use rudimentary weapons to maximise their impact."

"It again raises the question of whether a similar attack could happen in Seattle or San Diego, Miami or Manhattan," he said.

Beware of homegrown terrorists

Mueller added that although al-Qaeda remains a threat to the United States, US officials "must also focus on less well-known terrorist groups, as well as homegrown terrorists.

"And we must consider extremists from visa-waiver countries, who are merely an e-ticket away from the United States."

The FBI is "increasingly concerned with pockets of people around the world that identify with al-Qaeda and its ideology. Some may have little or no actual contact with al-Qaeda."

He also warned of homegrown threats: "A man from Minneapolis became what we believe to be the first US citizen to carry out a terrorist suicide bombing.

"The attack occurred last October in northern Somalia, but it appears that this individual was radicalised in his hometown in Minnesota," Mueller said.