The plaintiffs - including Chubb Corp., One Beacon Insurance Group and Crum and Foster Insurance Co. - said they had paid out $4 billion in property and workers' injury claims stemming from the attacks.

They name dozens of defendants in their complaint including Iran, Iraq, Sudan and Syria and Saudi Arabia.

In twin suits filed in courts in New York and Washington on Wednesday, they sought to recover their losses and win billions more in compensatory and punitive damages - for a total of $300 billion.

The insurers said al-Qaida and governments, organisations and individuals who gave it financial or political succour were responsible for the attacks and resulting losses.

Laws violated

"Absent the material support and resources provided by the co-defendants... al-Qaida would not have possessed the financial resources, physical assets, membership base, technological knowledge, communication skills, and global reach required to conceive, plan and execute the September 11th Attack," said their complaint.

Defendants violated a raft of laws including the Anti-Terrorism Act and the Torture Victim Protection Act, it further charged.

The legal offensive aims to "strike at the assets of sponsors of terrorism in order to put terrorism out of business," said Elliott Feldman of the Philadelphia law firm Cozen O'Connor, the plaintiffs' lawyers.

The lawsuits were filed one day after a judge in New York ruled families of victims of the attacks could sue for damages from airlines, airport security officials, a plane manufacturer, and the public entity that owned the World Trade Centre.