WASHINGTON (Reuters) - The U.S. military announced on Monday an anti-mine exercise in "the Middle East's international waterways" in May with more than 20 nations participating, the latest show of global will to keep waterways open as tensions with Iran simmer.

The drill was characterized as defensive and a follow-up to the IMCMEX 12 exercise held last September, focused on keeping oil shipping lanes open by clearing mines that potentially Iran, or even guerrilla groups, might deploy to disrupt tanker traffic.

"This year's effort will reaffirm the ongoing, global cooperation that this mission enjoys with the international community's strong support for free trade," General James Mattie, commander of the U.S. military's Central Command, said in a statement.

U.S. officials have in the past sought to play down any link between the drills and ongoing tensions with Iran, which is pushing forward with a nuclear program the West fears is aimed at giving it the capability to build an atomic bomb. Tehran says the program is peaceful.

But the anti-mine exercises are widely seen as a clear show of determination by a broad coalition of states to counter any attempt Iran might make to disrupt Gulf shipping in response to an Israeli or U.S. strike on its nuclear facilities - a form of retaliation Iran has repeatedly threatened.

Central Command said the drill in May, IMCMEX 13, would focus on mine countermeasures, as well as maritime security operations and protecting maritime infrastructure. It did not provide further details.

(Reporting by Phil Stewart; Editing by Todd Eastham)