A Chinese court may have secretly tried and sentenced the high-profile Uighur scholar Ilham Tohti, the lawyer hired to represent him has said.

Li Fangping said via his microblog that he had heard that a secret hearing had taken place but could not yet confirm it. He told the Guardian that two separate sources had informed him where the case took place and how many years Tohti was given, adding that the sentence was "heavy". He declined to give any further details.

Tohti was detained in January and formally charged the following month, the accusations variously reported as separatism or inciting separatism. His wife described the allegations as ridiculous and supporters say the case is retaliation for his outspokenness, noting that while he has boldly criticised government policies he has never called for independence for the north-western region of Xinjiang.

Separately, the state news agency Xinhua announced late on Monday that 13 people had been executed in Xinjiang for terrorist and other violent crimes.

It said three of those involved had organised and led attacks on a police station, hotel, government office and other locations, killing 24 police officers and civilians at Lukqun, Turpan, last year.

Salman Rushdie and other writers have urged the release of Tohti, noting in an open letter that his Uighur Online website was founded "with the express purpose of promoting understanding between Uighurs and Han Chinese".

The offence of separatism can carry the death penalty in "particularly vile" circumstances, while inciting it can mean jail for up to 15 years.

Tohti is not believed to have seen Li since his detention in Beijing by more than 30 police officers. He is believed to have been transferred to Xinjiang shortly after his arrest.

The Xinjiang government propaganda office said it had no information, while calls to the justice ministry rang unanswered.

"Under the law, a lawyer is supposed to be notified of a trial three days beforehand, regardless of whether it is open to the public or not," said Joshua Rosenzweig, an independent human rights law scholar in Hong Kong.

But he noted that it was possible another lawyer might have been appointed to handle Tohti's case. In some sensitive cases, authorities have chosen lawyers for detainees, while lawyers hired by their families have been told they cannot represent the individuals.

"Most of what we find out about these [separatism] cases is after the fact. There is relatively little reporting in state media ahead of time or at the time and rarely first-hand accounts by people who were in attendance. Most of the cases involving separatist charges that we know about are probably not reported by state media at all, but through unofficial channels," Rosenzweig said.

But he added: "I suspect in this case, given his profile, that if and when there is a verdict the state media will have some form of report."

Maya Wang, of Human Rights Watch, said that if Tohti had been tried and sentenced in secret it was "very, very disturbing". Even if a trial was yet to occur, she noted, the denial of access to the lawyer hired by his family was of grave concern.

She added: "He has always been one of the most moderate but vocal voices from Xinjiang. The charge of separatism is a far cry from what he is actually advocating, but in the context of Xinjiang, there has been a history of the Chinese government conflating peaceful advocacy for Uighur rights with charges of separatism and even terrorism."

Tohti's case comes amid growing concern about tensions in Xinjiang. There has been a string of deadly attacks in the last year, including two bombings in Urumqi and knife attacks in Kunming and elsewhere.

Before his detention the academic had raised questions about official accounts of what happened in Tiananmen Square last October when a car ploughed into crowds, killing two tourists and its three occupants.

On Monday, a court in Xinjiang sentenced three people to death and gave five others jail terms over that case. Husanjan Wuxur, Yusup Umarniyaz and Yusup Ahmat were given the death penalty for organising and leading a terrorist group and endangering public security with dangerous methods.

The court was told the three men and the driver of the car, Usmet Hasan, had watched terrorism videos and jointly planned terrorist acts. The remaining defendants were jailed for joining a terrorist group.

China has blamed the spate of attacks on extremist groups seeking an independent Xinjiang – which they call East Turkestan – working with foreign terror networks.

Uighur exiles and analysts argue that violence is being driven by resentment at Han migration, the economic marginalisation of Uighurs and restrictions on their culture and religion.