AMSTERDAM, Aug 12 (Reuters) - Security measures in and around Amsterdam's Schiphol airport have been extended again, probably to last throughout the holiday season, and will include soldiers helping border police check cars and trains.

Extra security around Schiphol started on July 30, although the Netherlands' Coordinator for Security and Counterterrorism has kept the nation's threat level at "substantial", one notch below the highest level of alertness for possible attacks.

Officials declined to elaborate on the nature of the threat, which they have described as a "signal" or "indication."

"We still cannot elaborate on the nature of the information that we have," Adam Elzakalai, the mayor of Haarlemmermeer, the municipality which oversees Schiphol, said.

He said he understood public frustration over the secrecy but it was "necessary to protect our sources and preserve our informational advantage".

Travellers to the airport are advised to expect delays of about half an hour.

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 in Syria and Iraq. (Reporting by Toby Sterling; Editing by Louise Ireland)

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