By Robert-Jan Bartunek

BRUSSELS (Reuters) - A Belgian judge on Tuesday sentenced 15 members of an Islamic State (IS) militant cell - nine of them in absentia - to up to 16 years in prison after convicting them of plotting attacks on targets including Brussels airport.

IS suicide bombers killed 16 people at Brussels airport - as well as 16 on a Brussels metro train - on March 22 this year, although prosecutors said the men sentenced on Tuesday had no role in those attacks.

The six defendants present in court on Tuesday were arrested after a bloody police raid in the town of Verviers at the start of 2015 that suggested Belgium had become a hub for militants returning from stints with Islamist militant groups fighting in Syria's civil war.

Prosecutors said the Verviers group were also in constant contact with Abelhamid Abaaoud, an Islamic State figure who planned several IS attacks in Europe including the bombing and shooting rampage in Paris that killed 130 people last November.

Abaaoud was shot dead in a gunbattle with French police five days after the Nov. 13 attack in Paris.

Plans found in 2015 on a mobile phone belonging to the Islamic State cell in Verviers included drawings of an airport and a baggage trolley with the word "bomb" written on it, prosecutors testified.

"For Abaaoud and the Verviers cell, the (Brussels) airport at Zaventem constituted a target," said Judge Pierre Hendrickx. "The cell in Verviers aimed for the bloodiest possible attacks using bombs and automatic weapons."

Belgian officials pointed to the police raid on the militant safe house in Verviers in January 2015 as a success story for the country's spy services. But French and Belgian police drew widespread criticism for security lapses seen as having made it easier for IS to wreak carnage in Paris and Brussels.

Marouan El Bali, who survived a gunbattle in Verviers in which police shot dead two other men present in the house at the time, was given 16 years in prison and a fine on Tuesday.

"He was in touch with leading figures in Syria," Hendrickx said. "This shows his true role within the group."

El Bali's lawyer said the convicted man was a childhood friend of one of the men who was killed by police in Verviers and could not be considered a leader of a militant group.

Two other convicted members of the cell, Mohammed Arshad Mahmood Najmi and Souhaib El Abdi, who were not in the house at the time of the raid, were also given 16-year sentences for their leading role in the cell.

Out of the 15 people sentenced on Tuesday, only six were in court. Nine others were subject to an international arrest warrant, a spokesman for the court said.

(Additional reporting by Julia Fioretti; Editing by Mark Heinrich)