Factbox: Attacks, incidents heightening Gulf tensions since May

DUBAI (Reuters) - Yemen's Iran-aligned Houthi group said it attacked two plants at the heart of Saudi Arabia's oil industry on Saturday, knocking out more than half the Kingdom's output.

The movement had threatened in May to attack 300 military targets in Saudi Arabia, the United Arab Emirates and Yemen.

That statement coincided with worsening tensions between Iran and the West after U.S. sanctions on Iranian oil exports took full effect in May.

Here are some of the documented attacks on Saudi Arabia claimed by the Houthis, and a number of incidents involving oil tankers, most of which Western officials have attributed to Iran. Tehran has denied any involvement.

-- The U.S. Maritime Administration on May 10 warns of possible Iranian attacks on shipping in the Gulf.

-- Four ships, including two Saudi oil tankers, are attacked on May 12 in the Gulf just outside the Strait of Hormuz, a major oil shipping route. U.S. officials pin the blame on Iran, a charge Tehran denies.

-- Saudi Arabia said armed Houthi drones struck two oil pumping stations in the kingdom on May 14, causing a fire which was contained and minor damage at one pump station, but did not disrupt oil output or exports. Saudi Aramco said it temporarily shut the East-West pipeline, known as Petroline, to evaluate its condition.

-- A Houthi drone on May 21 struck an arms depot in Saudi Arabia's Najran airport near the Yemeni border, causing a fire. The Saudi-led military coalition said a civilian facility in Najran province was attacked with an explosive-laden drone.

-- Houthis drones struck Abha civilian airport in southern Saudi Arabia on June 12, wounding 26 people.

-- Two tankers are attacked south of Hormuz on June 13. Washington again blames Iran and Tehran denies any role.

-- The Houthis launched a projectile on June 19 targeting a power transformer station near a desalination plant in Shuqaiq in southern Saudi Arabia. The strike led to a small fire that was quickly doused and did not impact operations.

-- Iran shot down an unmanned U.S. surveillance drone on June 20.

-- The Saudi-led military coalition said the Houthis launched an attack on Abha airport on June 23, killing one person and wounding 21 others.

-- British Royal Marines seized Iranian supertanker Grace 1 near Gibraltar on July 4 on suspicion of violating European Union sanctions on Syria.

-- British warship HMS Montrose on July 10 issued verbal warnings and aimed guns at boats believed to belong to Iranian Revolutionary Guards which approached BP (BP.L) oil tanker British Heritage at the northern entrance of Hormuz.

-- The United States said USS Boxer destroyed an Iranian drone on July 18 in the Strait of Hormuz after it threatened the U.S. Navy ship. Iran denied it has lost a drone.

-- Iran seized British-operated oil tankers Stena Impero and Mesdar on July 19, the latter released after receiving a warning from Iranian authorities.

-- Gibraltar freed the Grace 1, renamed the Adrian Darya 1, on Aug. 15 after receiving assurances from Tehran that the ship would not discharge its oil in Syria. On Sept 8 Iran said it unloaded its cargo somewhere on the Mediterranean coast. On Sept 11 Iran said the vessel had been sold at sea to a private company and denied Tehran had broken the assurances.

-- The Houthis claim responsibility for Sept 14 drone attacks on Saudi Aramco plants in Abqaiq and Khurais. The plants lie in the heartland of Saudi Arabia's oil industry, including the world's biggest petroleum processing facility.