The BBC's Lisa Upton: "Officials say the effect of the sanctions are likely to be modest"

The American Federal Bureau of Investigation put Osama bin Laden on its 10 most wanted list last month for his alleged involvement in the bombing of two US embassies in Africa last year and other violent acts aimed at the US.

The US launched a cruise missile attack against his Afghan base last year, but the Taleban say they now do not know where he is.

The US believes the Taleban harbours Osama bin Laden

US officials believe he remains in Afghanistan. In a written statement, President Bill Clinton said the imposition of sanctions was intended to deepen the international isolation of the Taleban and to limit its ability to support what he called terrorist networks.

But the leader of the Taleban, Mullah Mohammad Omar, brushed aside the move. "We don't care about any American sanctions," he told the Afghan Islamic Press (AIP).

Mullah Omar said he did not know where Osama bin Laden was; his government had neither asked him to leave Afghanistan, nor asked him to stay there.

Extradition requests

A State Department spokesman, James Foley, said the Taleban had repeatedly refused American requests to extradite Osama bin Laden for trial, and appeared committed to providing him with a safe haven.

Six US embassies in Africa were temporarily closed last week because of concerns that they might be in imminent danger from attack.

The new sanctions are aimed at increasing the pressure on the Taleban to hand over Osama bin Laden, by denying them access to American financial institutions and companies, and freezing any assets they may have in the US.

But Administration officials admit that the effect of the sanctions is likely to be modest given the fact that trade between the two countries amounted to only $24m last year.