An Iranian appeal court has upheld jail terms against two renowned doctors accused of involvement in an alleged US-backed plot to topple the Islamic regime, their lawyer said on Tuesday.

Brothers Arash Alaie and Kamiar Alaie, who are well known for their pioneering work in HIV/AIDS, had been sentenced to six and three years terms respectively by a lower court in January last year.

"In an extraordinary session on 18 March, the appeal court unfortunately confirmed (the sentences)," lawyer Massoud Shafiie told AFP.

Shafiie said the appeal court gave its verdict very quickly despite the voluminous case papers.

"It should have taken much longer to study in detail the arguments against the verdict. Therefore this can't be an appropriate decision," he said.

In January 2009, Iran announced it had dismantled a network allegedly funded by the United States to overthrow the Islamic regime by triggering social upheaval.

It said the two brothers were part of that network, along with two other suspects whose fate in not known.

Shafiie said he will appeal again to judiciary chief Ayatollah Mahmoud Hashemi Shahrudi.

"I am somehow sure that the verdict will change," Shafiie said.

Tehran regularly accuses Washington and London of covertly backing violent and non-violent action against the state in a bid to topple the hardline Islamic regime.