Belgium on Tuesday charged two men over the 2015 attack on a high-speed train bound for Paris in which an Islamic State group extremist shot and seriously wounded a passenger before being overpowered.

The pair, suspected of helping Moroccan gunman Ayoub El Khazzani, were charged with "participation in the activities of a terrorist group", Belgian prosecutors said in a statement.

They were among four men held on Monday as part of the probe into the August 2015 attack, which could have been a bloodbath but for the actions of quick-thinking passengers -- including two off-duty US servicemen -- who overpowered the heavily-armed Khazzani as he opened fire.

The prosecutor's office identified the pair who were charged as "Mohamed B., born on 10/04/1987" and "Youssef S., born on 29/08/1988". They are both Belgian.

A source close to the case told AFP the first man was Mohamed Bakkali, who is already in custody charged with helping to organise the November 2015 Paris attacks.

Thalys attacker Khazzani, a Moroccan who fought for IS in Syria, has told investigators he was acting on the orders of Abdelhamid Abaaoud, one of the organisers of the Paris attacks, whom he met in the war-torn Middle Eastern country.

Prosecutors said "no explosives nor weapons were found" during Monday's raids, one of which was in the gritty Brussels district of Molenbeek. The district gained notoriety as a hotbed of international jihadists after the attacks in the French and Belgian capitals.

The other two men arrested on Monday were released after "thorough questioning", Belgian prosecutors said.

Source: AFP