By Holly Williams BBC correspondent in Beijing

China has reacted angrily to US sanctions on Chinese firms accused of selling chemical and germ warfare materials to Iran.

The United States has placed sanctions on nine Chinese companies.

The US Government said the companies sold goods and technology to Iran that were then put to use in the country's chemical and conventional weapons programmes.

Sanctions

Affected companies may not do business with US firms Affected companies may not buy US goods

China complains that the sanctions are unreasonable and says it is working to prevent arms proliferation.

The issue of arms proliferation is the perennial irritant in US-China relations.

This is just the latest in a string of accusations by the US and denials by the Chinese.

The sanctions mean that the nine Chinese firms will not be able to do business with American partners or purchase materials from US companies.

But the US State Department has not yet released the names of the accused sellers.

Earlier accusations

In May this year, the US imposed sanctions on a further eight Chinese firms.

They were also accused of transferring destabilising arms and technology to Iran.

Last year, the US Government handed out the same punishment to a Chinese company that allegedly sold missile components to Pakistan.

China says it is tackling the problem of proliferation and points to new export controls.

But the US says the Chinese Government is not doing enough to stop further sales.