A group of Sudanese military officers were sentenced to between two and five years in prison on Sunday for their role in a coup attempt last year, the army and a defence lawyer said.

"The accused persons were convicted of attempting to undermine the constitutional and security system and threaten the country's unity and harm the armed forces by the use of force," army spokesman Sawarmi Khaled Saad said.

In addition to jail time, they were dismissed from military service but have 15 days to appeal the verdicts, he added, quoted by state news agency SUNA. He said 11 army men were convicted while one was acquitted.

A lawyer for the accused, Hashiem al-Jali, confirmed the sentences for a coup attempt last year but told AFP that only nine soldiers were convicted after the trial that lasted about three weeks.

Magdi El Gizouli, a fellow at the Rift Valley Institute, said it was "a political statement not a court ruling," while other analysts said the case revealed turmoil within the regime of President Omar al-Bashir, who himself seized power in a coup 24 years ago.

Jali said the heaviest sentence of five years was handed down to Brigadier Mohammed Ibrahim, who played a role in the 1989 coup.

He called it a "heavy punishment" for all the soldiers and said the defence team would look at ways of appealing.

But a regional political expert told AFP the penalty was in fact relatively light as they could have faced the firing squad.

"It means that President Bashir would like to calm the situation," said the expert, asking for anonymity.

"The regime is scared about (its) opponents."

When the trial began, he said that the case against the officers was "not very clear" and the evidence scant, with officials releasing only vague details about the plot.

"The problem is that the people who did this ... were hardcore supporters of Bashir, once upon a time," Gizouli said.

Most of the detainees are close to a vocal group of former volunteer mujahedeen fighters -- veterans of the 1983-2005 civil war -- and an elite group within them called Al-Saihun or "tourists for the sake of God".

The soldiers were arrested in November along with the country's former intelligence chief in a plot which analysts said was linked to hardcore Islamist officers.

Along with a youth movement within the ruling National Congress Party, the war veterans have called for new national leadership and a return to Islamic values because they said the government is tainted by corruption and other problems.

The accused plotters were detained without any shots being fired.

Authorities also arrested Salah Gosh, who served as national intelligence boss until 2009. As he is not a military officer, his case was to be heard later.

Sudan has experienced at least seven coups or attempted coups in its 57-year history.

Some tension has eased in the country since early March when Sudan and South Sudan finally settled on detailed timetables to implement crucial economic and security pacts, after months of intermittent border clashes.

Bashir announced last week that all political prisoners would be freed as the government seeks a broad political dialogue, "including (with) those who are armed".

Authorities then released seven political prisoners, though the opposition says hundreds more are still being held.