Witnesses say there has been a large explosion at the US embassy building in Beijing, China, with videos and pictures posted to social media showing a large cloud of smoke and what appear to be police vehicles surrounding the area.

Beijing police said the blast was caused by a small homemade explosive device, detonated by a 26-year-old man who hurt only himself.

It comes amid a number of conflicting reports, and after Chinese state media described the incident as "an attempt at self-immolation" and that a woman was in police custody.

A witness told Reuters a Chinese police car appeared to have been damaged in the incident.

The police statement identified the man only by his surname, Jiang, and said he was from the city of Tongliao in the Chinese region of Inner Mongolia. No other injuries have been reported.

On its official social media feeds, the US mission in China continued posting but made no mention of the blast, and calls to the media wing of the embassy went unanswered.

The Global Times, an English-language arm of the state-run People's Daily, had tweeted earlier: "Witness says police took away a woman spraying gasoline on herself in suspected attempt at self-immolation outside US embassy in Beijing around 11 am Thursday."

"Something just exploded at the US Embassy in Beijing, China just a few min ago. Smoke everywhere," wrote Jimmy Zhong on Twitter.