DUBAI (REUTERS) - The Iranian coast guard has seized two Saudi fishing vessels after they entered Iran's territorial waters, a local coast guard official was quoted as saying on Wednesday.

"Yesterday, the coast guard deployed in the country's southern waters came to spot two vessels in Iran's protected waters," commander of the Bushehr province coast guard Qalandar Lashkari told the Fars news agency.

He said the two vessels were fishing illegally in Iranian waters under Saudi flags. The nine sailors of various nationalities on board were arrested, he said.

Relations are tense between Shi'ite Muslim power Iran and Sunni Muslim-led Saudi Arabia, and seizures of vessels accused of straying into one another's waters occur a few times a year.

Saudi Arabia has accused Iran of fomenting unrest among Shi'ites in its oil-rich Eastern Province, and Riyadh fears Teheran's nuclear work is a cover for an atomic weapons programme. Iran denies both charges.

Despite its mistrust of Iran, Saudi Arabia gave a qualified welcome on Monday to Teheran's interim deal with world powers over its disputed nuclear programme.

Fars did not give a location for the incident, but the province of Bushehr stretches down Iran's northern Gulf coast and is the site of the only nuclear power station in the region.

In a similar incident in early January, Iran's coast guard detained 10 Indian nationals aboard two Saudi-flagged fishing vessels.

The arrests came just days after Saudi Arabia detained 21 Iranian nationals who were aboard two boats near al-Harqus island, 42 miles (68km) off the Saudi coast.