The bodies of 28 Sudanese army officers involved in a failed coup against former president Omar al-Bashir in 1990 have been found in a mass grave, prosecutors said on Thursday.

It is the second mass grave discovered since Bashir, who ruled the African country for three decades and ruthlessly crushed dissent, was overthrown by the army during street protests last year.

The bodies were found in Omdurman, twin city of the capital Khartoum, as part of investigations into misdeeds committed under his rule, public prosecutor Tagelsir al-Hebr said.

"It took three weeks and the work of 22 experts from different fields to locate it," he said in a statement.

"Operations are under way to dig up the bodies and medical experts will carry out tests to identify the remains."

In April 1990, the officers had surrounded an army headquarters and several barracks before being arrested and killed.

Bashir, 76, and many of his aides have been kept in Khartoum's Kober prison on multiple charges since being ousted.

Members of a forensic team at a first mass grave found south of Khartoum in June
Members of a forensic team at a first mass grave found south of Khartoum in June AFP / -

He has already been convicted of corruption and is currently on trial over the Islamist-backed 1989 military coup that brought him to power.

The International Criminal Court also wants to try Bashir for genocide and war crimes committed in the Darfur region, where 300,000 people were killed and 2.5 million others forced to flee.

Sudan has since August been led by a civilian-majority administration presiding over a three-year transitional period.

Authorities have vowed to probe acts committed under Bashir's rule as well as violence during last year's protests, which led to his removal from power in April 2019.

In mid-June, they uncovered a mass grave of conscripts allegedly killed after trying to flee a military camp in 1998.

The Sudanese government said at the time that around 55 young conscripts who fled the base drowned when their overloaded boat capsized in the Blue Nile.

But opposition groups accused the regime of deliberately killing the conscripts and reported a death toll of more than 100.

Bashir used conscripts in the civil war against rebels in the oil-rich south, which seceded in 2011. Many Sudanese families reported that their sons went missing and their remains were never found.