The lawyer for an American citizen accused of plotting a radioactive "dirty bomb" attack in the US has said he is being held illegally and the evidence against him is "weak at best".

Donna Newman submitted a petition in the Manhattan federal court on Wednesday on behalf of Jose Padilla, who also goes by the name of Abdullah al-Muhajir.

"There no longer exists probable cause to detain Padilla for any reason," the petition said.

Padilla was arrested on his return from Pakistan

"The evidence linking Padilla to the alleged 'dirty bomb' plot is weak at best. There is insufficient evidence for the government to obtain an indictment against Padilla."

US District Judge Michael Mukasey said the government had until 21 June to file a motion to dismiss the petition or transfer it to another jurisdiction.

Mr Padilla - who has been linked to al-Qaeda by US officials - was arrested on 8 May at Chicago's O'Hare international airport, but his detention was only announced by the government on Monday.

Military custody

US officials declared him an "enemy combatant" and he is now under military custody at a naval facility in South Carolina.

"Enemy combatant" status

Based on a 1942 Supreme Court ruling Suspect can be detained under military custody indefinitely Detainee does not have to be charged Suspect can be questioned more freely Q&A: Rights versus security

As an "enemy combatant" Mr Padilla - a 31-year-old US citizen of Puerto Rican origin - can be detained indefinitely without trial.

Ms Newman said she had not been allowed to speak to him since his transfer.

President George W Bush said on Tuesday that there was a "full-scale manhunt on" for anyone else believed to be involved in an alleged al-Qaeda plot to detonate a radioactive bomb in the US.

At least one associate of Mr Padilla has been detained and is being questioned in a foreign country, a US official said on Wednesday, giving no further details.

No trial yet

The US blames the al-Qaeda terror group for the 11 September attacks in New York and Washington.

"This guy, Padilla, is a bad guy. And he is where he needs to be detained," Mr Bush said.

US Defence Secretary Donald Rumsfeld said his government was "not interested in trying him at the moment".

Ashcroft: "Unfolding plot" to explode radioactive device

"We're not interested in punishing him... we're interested in finding out what in the world he knows," Mr Rumsfeld said.

Some Bush administration officials have indicated that Attorney General John Ashcroft overstated the "dirty bomb" threat when he announced Mr Padilla's detention.

"I don't think there was actually a plot beyond some fairly loose talk and (Padilla's) coming in here obviously to plan further deeds," Deputy Defence Secretary Paul Wolfowitz told CBS television.

In his announcement, Mr Ashcroft said the authorities had "disrupted an unfolding terrorist plot to attack the United States by exploding a radioactive 'dirty bomb'".

According to the newspaper USA Today, the announcement "surprised the White House and law enforcement officials".

"They say that the former Chicago gang member once known as Jose Padilla was up to no good, but that any plans involving radiation were not as mature as Ashcroft suggested," the paper said.