Iranian authorities have imposed a 28-year sentence on jailed Belgian aid worker Olivier Vandecasteele, a spokesman for his family said Wednesday.

The 41-year-old was arrested in Iran at the end of February and was at one point being held in Tehran's notorious Evin prison, reportedly suspected of spying.

Belgium and Vandecasteele's family insist he is innocent, effectively held as a hostage in Tehran's efforts to force Belgium to release an Iranian agent convicted of terrorism.

"The family are devastated," spokesman Olivier Van Steirtegem told AFP, after the Belgian government informed the family of the news.

"Can you imagine? If there's no solution he could stay in prison until 2050. He'll be almost 70," he said, urging Belgium to find a way to revive a prisoner swap treaty.

Van Steirtegem said that Vandescasteele's family had been invited to meet Belgium's Prime Minister Alexander De Croo and several ministers on Tuesday.

Justice Minister Vincent Van Quickenborne, the family ally said, had received a call from his Iranian counterpart communicating the court's verdict -- but that they had no details on the charges.

News of Vandecasteele's sentence, which has not been publicly confirmed by Iranian authorities, will revive debate in Belgium over a prisoner exchange treaty with Iran.

De Croo's government has described this in the past as the only option for a transfer, and the family spokesman told AFP that this remained the position at Tuesday's meeting.

"There's no Plan B," he noted.

Last week, Belgium's constitutional court suspended the controversial treaty pending a final ruling on its legality in three months.

Opponents of the Iranian government challenged the deal, which they argue was "tailor-made" to permit the release of Assadollah Assadi, an Iranian diplomat sentenced last year to 20 years in prison.

An Antwerp court convicted Assadi of supplying explosives to a couple from Belgium who were to travel to Paris to target a meeting of Iran's exiled opposition.

Iran reacted with fury to the sentencing and the stalled prisoner exchange treaty was proposed as a way to win Vandecasteele's release, despite concerns it would be seen to reward hostage-taking.