A Chinese appeal court on Wednesday upheld a five-year jail term handed to an activist who was probing whether shoddy construction caused school collapses in a massive 2008 quake, his lawyer said.

The sentence for "inciting subversion of state power" handed to writer and environmental campaigner Tan Zuoren related to several articles he published online about Beijing's brutal crushing of the 1989 Tiananmen protests.

But Tan was arrested last year as he was investigating the deaths of thousands of children when their schools collapsed in the May 2008 quake in the southwestern province of Sichuan, which left nearly 88,000 people dead or missing.

"The trial only lasted 12 minutes. The court confirmed the verdict handed down by the lower court," Tan's lawyer Pu Zhiqiang told AFP by telephone from the Sichuan capital Chengdu.

Rights watchdog Amnesty International - which suggested at the time of Tan's trial that his independent quake probe, not his Tiananmen writings, was probably the real reason for his detention - denounced Wednesday's decision.

"This is a politically motivated outcome of a grossly unfair legal process," said Catherine Baber, Amnesty's deputy director for the Asia-Pacific.

Phelim Kine, an Asia researcher for New York-based Human Rights Watch, told AFP that the court in Chengdu had missed an opportunity to correct a miscarriage of justice.

"The Chinese constitution guarantees citizens' rights to the freedom of expression, but the government and its politicised judiciary appear to have again denied that right to an outspoken civil society activist," Kine said.

At Tan's trial last August, controversial Chinese artist Ai Weiwei - who also investigated the quake school deaths - said he was detained and beaten by police who blocked him from testifying on Tan's behalf.

Ai underwent surgery in Germany the following month to relieve pressure on his brain from a blood clot which he said was the result of the police beating in Chengdu.

Schools bore the brunt of the Sichuan quake, with thousands collapsing on top of students, fuelling angry charges from parents that corruption had led to shoddy construction.