Traffic through the Channel Tunnel was halted on Saturday after smoke was detected from an unknown source, said Eurotunnel, the operator of the crossing between Britain and France.

Eurostar, which operates train services through the tunnel between Paris, London and Brussels, said separately on its website that no trains could run on Saturday following the closure.

France has been on high alert since Islamist militants killed 17 people in three days of violence in Paris that began on Jan. 7 with an attack on the offices of a satirical newspaper.