Belgian police have detained 16 people in anti-terror raids in several cities.
The raids were linked to two separate investigations into Chechen extremists in Belgium, a federal prosecutor said.
Some suspects were linked to al-Nusra Front in Syria and the Islamic Caucasus Emirate, active in southern Russia.
There were 10 arrests in and around Ostend on the coast. Other raids took place in Antwerp, Louvain, Bredene and Namur. Some suspects allegedly tried to recruit Belgians for "jihad" in Syria.
Some of those investigated had received jihadist training and had travelled to Syria, Afghanistan and Chechnya.
The suspects were said to have been in two groups, one based on the coast, the other inland around Louvain.
One of those detained in the Ostend area was said to have been wounded in fighting in Syria and had returned for treatment.
The Louvain group was believed to have been plotting a terror attack in Belgium.
Both groups were said to have communicated using the internet messaging service, Whatsapp, and investigators said US authorities had given them help in tracing the messages.
Compared with other EU countries Belgium has a very high proportion of jihadists who have gone to join militants in Syria and Iraq.
In January, Belgian police were involved in a shoot-out in Verviers, near the German border, in which two suspected jihadists were shot dead.
Thirteen suspects were arrested in that operation. It came soon after the Paris terror attacks on Charlie Hebdo cartoonists and a Jewish supermarket, in which Islamist gunmen killed 17 people.