Acting U.S. Defense Secretary Mark Esper says he came away with no firm commitments from NATO allies to participate in a global effort to secure international waterways against Iran.

He says the U.S. will provide more details to allies next month on how the Iranian threat has escalated and how they can work together to deter further aggression.

The U.S. has blamed Iran for recent attacks on oil tankers near the Persian Gulf.

A senior U.S .official says allies expressed interest in Esper's request during their meetings in Brussels. The official spoke on condition of anonymity to describe private discussions.

NATO allies have expressed reluctance to get involved in any military effort to help secure the region. Europe wants more emphasis on minimizing the chances of war.