AMSTERDAM (Reuters) - Dutch authorities said on Monday they will extend extra security measures around Amsterdam's Schiphol, due to a heightened threat level at one of Europe's largest airports.

Police have been searching vehicles on major roads to the airport since Saturday.

No details have been provided about the perceived threat, but the Netherlands remains one notch below the highest level of alertness for possible attacks.

"The extended security (measures) will remain in place for the time being," a statement by local authorities said. The measures would increase safety for passengers and personnel at the airport, they said.

All travellers should count on possible delays due to spot checks on trains and vehicles, they said.

Earlier on Monday, all traffic to the airport was halted and police detained a man who spoke of a bomb a bus. No explosives were found.

After attacks by Islamist militants in France, Belgium and Germany, the Netherlands is considered a potential target, because it supports U.S.-led military operations against the Islamic State jihadist militia in the Middle East.

(Reporting by Anthony Deutsch; Editing by Richard Balmforth)