Belgian police shot dead two suspects in an operation in the eastern town of Verviers on Thursday, aimed at thwarting a major attack that officials said could have been launched within hours.

Spokesmen for the federal prosecutor's office told a Brussels press conference a few hours later that the suspects had shot first.

"During the search, certain suspects immediately opened fire at special forces of the police with automatic weapons. They opened fire for several minutes. Two suspects were killed and a third was arrested," Thierry Werts said.

"This operational cell of about 10 people, some of whom had returned from Syria, was on the point of launching significant terrorist attacks in Belgium," he added.

This raid was just one of around a dozen conducted by police in Verviers and the greater Brussels region lasting into the early hours of Friday.

Police facilities among the targets?

Among the targets the suspects were planning to hit are believed to have been police facilities - part of the reason that the authorities on Thursday raised the security alert level from two to three, the second highest level.

"It shows we have to be extremely careful," another spokesman for the prosecutor's office, Eric van der Sypt said, adding that the Verviers suspects were "extremely well-armed men" equipped with automatic weapons.

After holding an emergency meeting with his security chiefs, Prime Minister Charles Michel described the decision to increase the threat level as a "choice for prudence."

At the same time though, the prime minister noted that there was "no concrete or specific knowledge of new elements of threat."

He also said Thursday's raids were a sign of Belgium's "determination to fight those who want to spread terror."

The raids came with Europeans on edge just days after terrorist attacks in France that left 17 people dead. However, police have not linked the suspects in the Belgian raids to the Paris attacks.

In a separate development in Belgium, police announced earlier on Thursday that they had arrested a man in the southern city of Charleroi, who they believe may have supplied the gunman in the deadly hostage-taking in a kosher supermarket in Paris last Friday.

German police also carried out counter-terrorism operations on Thursday.

pfd/rc (AFP, AP)