The man, known as Abu Naba, was detained in October as he was "getting into a taxi, just after landing in Baghdad", CTS spokesman Sabah al-Noaman told AFP.

Describing him as a "major target", Noaman said Abu Naba had been steering financial support to IS, organising meetings and relaying messages between jihadist members.

"He began his jihadist path in 2003 with Al-Qaeda, before joining various groups that eventually led to IS," he said.

But Noaman declined to reveal Abu Naba's real name, where he had been flying in from and how he managed to cross through airport security before he was apprehended.

In 2014, the ultra-conservative and violent faction seized a third of Iraqi territory, which local troops backed by the US-led coalition only recaptured in late 2017.

More than a year later, in 2019, IS lost its last foothold in neighbouring Syria.

Jihadist sleeper cells have continued to wage hit-and-run attacks, including one north of Baghdad late Saturday that killed six security forces and four civilians.

They have also continued to transfer funds and personnel across Iraqi territory, Iraqi and Western officials say.

"Abu Naba had been in contact with remaining members of IS in Iraq, and we were monitoring their conversations for a long time," said Noaman.

Since his arrest, Abu Naba has remained in Iraqi custody and is being interrogated.

Noaman said he would be tried under Iraq's counter-terror law, which carries the death penalty for "membership in a terrorist organisation".

Iraq ranks fifth among countries that carry out death sentences, according to Amnesty International, which documented 100 executions in the country in 2019.

Since declaring IS defeated in 2017, Iraq has sentenced to death hundreds of its own citizens for membership in the jihadist faction but only a small proportion have been carried out.

On November 16, 21 men convicted of "terrorism" charges were hanged at the notorious Nasiriyah prison in southern Iraq, which the United Nations warned was "deeply troubling."