TEHRAN — Saudi Arabia said on Monday that its navy had seized three members of Iran’s Revolutionary Guards piloting a boat loaded with explosives toward a Saudi offshore oil drilling rig.

The claim was not confirmed but threatened to further fray relations between the rival powers, which have accused each other of fomenting terrorism and instability against a growing backdrop of tensions roiling the Middle East.

Iran denied the Saudi claim and accused the Saudi Navy of having shot at boats belonging to “simple fishermen” from Iran’s southern Persian Gulf port of Bushehr in an unprovoked attack that had left one Iranian dead.

About the only thing both sides appeared to agree on was that the episode happened on Friday, when Iran’s state media first reported its version of events.