US President Donald Trump ordered some 1,500 troops to the Gulf region for "mostly protective" purposes on Friday.

He made the announcement to reporters on the White House lawn before boarding Marine One.

At a Pentagon briefing on Friday, Director of the Joint Staff Vice Admiral Michael Gilday would not say where the additional troops would be sent, other than that they would not be heading to Iraq or Syria.

Their mission will be to work on missile defence systems and intelligence surveillance using radars and drones.

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