Saudi Arabia has detained 21 Iranian nationals who were aboard two boats near an island off the kingdom's eastern coast, the Saudi border guard said on Thursday.
Separated by about 250km of Gulf waters, Shi'ite Muslim power Iran and Sunni-led Saudi Arabia have often had tense relations.
US ally Saudi Arabia has accused Iran of fomenting unrest among Shi'ites in its oil-rich Eastern Province, a charge Iran denies.
"One of the marine patrols was able to seize two Iranian boats on Wednesday inside Saudi regional waters," a border guard statement said.
"One of the boats had 12 people and the other nine people ... all of them are Iranians and are being questioned."
The Iranians were seized near al-Harqus island, 78km off the Saudi coast, the statement said but gave no further details.