Sharif al-Masri, who was captured late August near Pakistan's border with Iran and Afghanistan, has told interrogators of "al-Qaeda's interest in moving nuclear materials from Europe to either the US or Mexico," according to a report circulating among US government officials, the weekly magazine reported.

Osama bin Laden's network has considered plans to "smuggle nuclear materials to Mexico, then operatives would carry material into the US," Masri said, according to the report, parts of which were read to Time.

Though unproved, Masri's account has added to US concerns over its border with Mexico, the magazine said.

US and Mexican intelligence officials have also discussed reports from several Al-Qaeda detainees saying that Mexico could potentially be used as a staging area "to acquire end-stage chemical, biological, radiological and nuclear material."

US officials are closely monitoring heavy trucks crossing the border, while Mexicans will watch flight schools and aviation facilities in Mexico.

Some senior US officials are worried about the theft in southern California of a crop duster plane that was seen flying south toward Mexico two weeks ago.

Though it is unclear whether the theft is connected to terrorism, a senior US law enforcement official told Time crop dusters can be used to disperse toxic agents.