The United States is seeking the extradition of the leader of Colombia's right-wing paramilitary group, the United Self-Defence Forces of Colombia (AUC).

The US Attorney General, John Ashcroft, described the paramilitary leader, Carlos Castano, as a violent drug trafficker who threatened America's security.

This will be a test of the government's commitment to battle the rebels

Mr Ashcroft called on Mr Castano to turn himself in to the Colombian authorities and face extradition - something which local media is reporting he has offered to do.

Mr Castano told national television and radio broadcast network RCN that his lawyer had gone to Washington to arrange his surrender.

Colombian President Alvaro Uribe is also currently in the US, hoping to rally support and funding for his promised war against illegal armed groups and drug-traffickers.

The AUC is accused of carrying out some of the worst atrocities in Colombia's long civil-war and is on Washington's list of terrorist organisations.

Drugs shipments

Mr Castano's group, which numbers at least 8,000, is fighting two sets of left-wing rebels.

Announcing details of the indictment Mr Ashcroft said Mr Castano had directed the production, sale and shipping of cocaine, 17 tonnes of which were allegedly sent to the US, with the proceeds funding militant activity.

The US is also seeking the extradition of Mr Castano's military commander Salvatore Mancuso and group member Juan Carlos Sierra-Ramirez.

Mr Ashcroft described all three as violent criminals whose drugs poisoned citizens and threatened America's national security.

During the news conference, Mr Ashcroft praised President Uribe for his commitment to the investigation.

Surrender plan

But the BBC's correspondent in Washington, Steve Kingstone, says that if there is to be an extradition, the initial impetus may come from Mr Castano himself.

Mr Ashcroft quoted a recent article from the Washington Post newspaper in which the rebel leader apparently offered to surrender and face US justice.

Observers say that whether Colombia's security forces capture Mr Castano or not will be a litmus test of the government's commitment to beat the paramilitaries.

Our correspondent says the Bush administration meanwhile is describing the plan to extradite Mr Castano as the convergence of its two top foreign policy priorities - the fight against terror and the international war on drugs.