Iran’s Revolutionary Guards have seized a Saudi Arabian fishing boat and arrested its crew, in the latest escalation of tension between Tehran and Riyadh.

A spokesman in Bushehr Province in southern Iran said five Indian nationals were detained on Friday after they crossed into Iranian territorial waters in the Persian Gulf. He said it was the second time in the past month that a Saudi boat and its crew had been intercepted.

Last month Saudi security forces captured an Iranian boat in the Gulf, killing one man and arresting three others. The Saudis claimed the men were working for the Iranian miltiary. Tehran said they were fisherman who had legal documents but lost their way.

Iran and Saudi Arabia have been long-time rivals in the region, but relations have plummeted since September 2015, when 464 Iranians were among thousands killed in a crush at the Mecca pilgrimage.

Three months later Saudi Arabia broke diplomatic links with the Islamic Republic following Riyadh’s execution of a prominent Shia cleric and a Tehran crowd attacking and burning the Saudi Embassy.

Despite efforts at mediation by other Gulf States, the situation has continued to worsen amid the involvement of both sides in the Syrian conflict, repression in Saudi ally Bahrain, and Saudi intervention in the Yemeni civil war. In May, the emerging force in Saudi politics, Crown Prince Mohammad bin Salman, said any battles would be inside Iran. Iranian leaders, including the Supreme Leader and the Revolutionary Guards, have responded with proclamation of a US-Saudi-Israeli axis trying to overthrow the Islamic Republic.

Last month Saudi Arabia and its allies broke diplomatic relations with Qatar and imposed an air and sea blockade, citing Doha’s support of Iran as one of the reasons.

Earlier this week Kuwait expelled 15 Iranian diplomats, as it asserted that Tehran had supported a “terrorist” cell broken up in 2015.

See Iran Daily, July 22: Kuwait Expels Iranian Diplomats