STOCKHOLM, Aug 21 (Reuters) - Iranian Foreign Minister Mohammad Javad Zarif said on Wednesday Tehran may act "unpredictably" in response to the United States' "unpredictable" policies under U.S. President Donald Trump.

"Mutual unpredictability will lead to chaos. President Trump cannot expect to be unpredictable and expect others to be predictable," he said in a speech at the Stockholm International Peace Research Institute (SIPRI).

He added: "Gulf Arabs cannot achieve security (in the region) by spending billions of dollars on purchasing Western weapons... No amount of foreign military presence (in the Gulf) can prevent insecurity." (Reporting by Simon Johnson in Stockholm; Writing by Tuqa Khalid in Dubai; Editing by Toby Chopra)