US commanders say the new members will provide personnel and patrol boats, but have not specified how many.
Qatar and Kuwait have joined a new naval task force patrolling the Gulf.
The UK is one of the largest contributors to the International Maritime Security Construct (IMSC), which was launched in Bahrain earlier this month.
Initially made up of seven nations, it was instigated by President Trump after a series of attacks on commercial shipping in the Gulf, which Washington blamed on Tehran.
These included the seizure of British-flagged tanker Stena Impero by Iran's Islamic Revolutionary Guard Corps in July.
With the admission of Kuwait and Qatar, the IMSC's membership has climbed to nine.
Other nations have been invited to join, but the response to date has been lukewarm.
The Royal Navy has four minesweepers based in Bahrain, as well as two frigates - HMS Kent and HMS Montrose - and destroyer HMS Defender protecting UK shipping from threats in the Gulf.
Reports suggest that plans for a French-led European mission to perform a similar deterrence role are as yet undeveloped.