The Netherlands has been at alert level "Alpha" since the Russian invasion of Ukraine, but Wednesday's increase took it to "Alpha Plus", meaning "increased threats" in Europe.

Alpha Plus is level two on a six-level rating system that goes up to level D meaning "a targeted attack is imminent or has taken place."

The raised threat level means military personnel should step up surveillance of suspicious people and vehicles, as well as drones, said the defence ministry.

The threat analysis from the MIVD spy agency is that "other countries are increasingly prepared to engage in sabotage activities, also near the Dutch border," the ministry said.

"The increase is also related to increased threats in Europe. There is currently no concrete threat against a military site," the statement added.