Saudi Arabia said on Monday that it captured three Iranian Revolutionary Guards aboard an explosive-laden boat heading to an oil platform in the Gulf, further ratcheting up tensions in the region.
Iran said the three people detained were fishermen.
The Saudi statement came three days after it said the incident occurred in the Marjan oil field, and two days after Iran accused the Saudis of shooting one of its fishermen in the Gulf waters which divide their two countries.
Relations between Saudi Arabia and Iran were already strained but escalated after Saudi Arabia, the United Arab Emirates and other allies cut ties with Qatar two weeks ago.
They accuse Doha of supporting extremist groups, including some backed by Iran, "that aim to destabilise the region".
Last week, Iranian foreign minister Mohammad Javad Zarif called for a permanent mechanism in the Gulf to resolve crises like the diplomatic and economic isolation of Qatar.
The three guards "are now being questioned by Saudi authorities," the information and culture ministry said in its statement.
"It is clear this was intended to be a terrorist act in Saudi territorial waters designed to cause severe damage to people and property," the ministry said.
In Iran, Majid Aghababaie, head of border affairs at the interior ministry, said the three detained were fishermen from the southern Iranian port of Bushehr.
"There is no proof that they are military personnel," he said, in remarks carried by the ILNA news agency.
Saudi Arabia has said it seized weapons from a boat captured in the Marjan field at about 8:30 pm on Friday.
It said the navy fired warning shots when three small boats entered Saudi territorial waters and headed at high speed towards the platforms.
That statement made no mention of explosives and did not detail what type of weapons were found, though it said they were for "subversive purposes".
Iraq, Sudan leaders visiting
That statement also made no mention of arrests but said the boats bore "red and white flags". Two of the boats got away, it said.
On Saturday, Iran accused the Saudi coastguard of killing one of its fishermen after two fishing boats may have strayed into Saudi waters.
Saudi Arabia regularly accuses Shiite-dominated Iran of interfering in Arab countries, and has suggested it is linked to instability in the kingdom's east, where minority Shiites live.
Iran's Revolutionary Guard Corps in turn has accused Saudi Arabia of involvement in June 7 attacks in Tehran, when gunmen and suicide bombers attacked the parliament complex and the shrine of revolutionary leader Ayatollah Ruhollah Khomeini.
Seventeen people were killed in the attack claimed by the Islamic State group.
Saudi Arabia's announcement of the Guards' capture coincided with visits to the kingdom by Iraq's Prime Minister Haider al-Abadi, and Sudan's President Omar al-Bashir.
Saudi Arabia has been trying to cement its ties throughout the Muslim world against the backdrop of higher tensions with its Iranian rival.
Iran said the three people detained were fishermen ||Saudi said it captured 3 Iran guards aboard an explosive-laden boat heading to an oil platform in the Gulf