Spielberg has been working for several months to help put together the opening ceremony for the Games, which are widely seen as a sort of coming-out party for China's emergence as a world power. For several months, too, he has come under pressure from Darfur activists, who have accused him of cosying up to the country most directly involved in trade with the Sudanese masterminds of the slaughter.

A spokesman for Spielberg says the director intended to continue applying pressure on China to change its policies, and is ruling nothing out - including withdrawal from his unpaid position as artistic adviser.

The threat did not appear to go unheard in Beijing, because China's special envoy on Darfur gave an interview to the official China Daily newspaper yesterday in which he defended his country's policies. He said there was no point imposing United Nations peacekeepers on an unwilling government in Khartoum because coercion "will lead us nowhere ... China insists on using influence without interference, and we know respect for all parties is vital to finding a solution," the envoy, Liu Guijin, said. There was no immediate response to this from the Spielberg camp, but it appeared to fall far short of what the Hollywood director was looking for.

Spielberg, who used the success of his Oscar-winning Holocaust drama Schindler's List to set up an educational foundation dedicated to teaching children about the genocidal crimes of the Nazis, wrote a letter to China's President, Hu Jintao, a few months ago urging him to change his position on Sudan.

He has also donated about $1m (£500,000) to aid groups working in Darfur to protect the predominantly non-Arab civilian population, which has become prey to pro-government Arab militias. International experts say somewhere from 200,000 to 500,000 people have been killed in the conflict, and more than two million displaced.

Spielberg's spokesman, Andy Spahn, told ABC News on Thursday that there had been further discussions and he very much hoped the Chinese would indicated some sort of shift in their position very soon. "Steven is one [of] many advisers to the Beijing Games and he is trying to use the Games to engage the Chinese on this issue," Mr Spahn said. "We are in the midst of that right now. We're engaged in a little bit of a back-and-forth private dialogue."

Mr Spahn said all options were on the table, including resignation, but that it was far from clear how best to apply pressure. "Our main interest is ending the genocide. No one is clear on the best way to do this," he added. Spielberg is one of a clutch of Hollywood celebrities who are deeply concerned about the genocide. The actor George Clooney has been perhaps the most active, travelling to the region with his journalist father and addressing the UN General Assembly. Clooney also recruited his co-stars on Ocean's Thirteen - Brad Pitt, Don Cheadle and Matt Damon - as well as the film's producer, Jerry Weintraub, to help raise $5m for aid agencies in Darfur. They are setting up an anti-genocide foundation, Not On Our Watch. The actress Mia Farrow, however, criticised Spielberg in an editorial for the Wall Street Journal in March, saying the director was in danger of becoming a modern-day Leni Riefenstahl - a reference to the brilliant Nazi-era director who acted as Hitler's chief celluloid propagandist.