Eight kg (17 lb) of radioactive uranium has gone missing in China, delaying the verdict in a trial of four men charged with attempting to sell it on the black market, state media said on Friday.

A court in Guangzhou, capital of China's southern province of Guangdong, heard the four tried to sell the material between 2005 and January 2007, the China Daily said.

The men were arrested in January after a potential buyer in Hong Kong reported them to the authorities, the paper said.

However, despite having the four men in custody, police were unable to locate the uranium.

"The men claimed it had been lost because it had been moved around so much between potential buyers," the paper said.

A verdict had yet to be reached "as the court said the trial would continue until authorities tracked down" the uranium.

More than 20 people had fallen sick after being exposed to the radioactive material, the Hong Kong-based Information Center for Human Rights and Democracy said, citing an official involved in the investigation.

Court documents identified it as fissile uranium-235, the Center said, adding that it originated in a mine in Hunan province that was open from 1958 to 1985.

Under Chinese law, the illegal trade in uranium carries a sentence of between three and 10 years in prison. In exceptional cases, it can carry the death sentence.

"The radioactive substance uranium does not explode when it is in its raw state, but it is very harmful to people's health," Jiang Chaoqiang, director of the Guangzhou No 12 People's Hospital, told China Daily.

"Therefore it needed to be found as soon as possible."