By Jon Leyne BBC State Department correspondent

The United States has imposed sanctions on 10 foreign companies accused of helping Iran or Iraq acquire weapons of mass destruction.

It is believed most of the sanctions have been imposed on Chinese companies, but this has not been officially confirmed.

The measures prevent the US Government from doing business with the companies and also stops the sanctioned firms from importing components from the US.

These new sanctions are part of a stepped-up effort by the Bush administration to prevent the spread of weapons of mass destruction.

Symbolic

The State Department has not given many details yet, but according to the Washington Times newspaper, it is mostly Chinese companies that are being punished for helping Iran develop chemical and biological weapons.

China denies breaking an anti-proliferation treaty

This is the fourth time in less than a year that the US has punished China for selling arms to Iran.

The Chinese companies have been penalised for selling both conventional weaponry and components for weapons of mass destruction.

It follows a number of years in which the Clinton administration was reluctant to take action.

Critics say the measures are still only symbolic, because companies can avoid the restrictions by trading through subsidiaries.