The United States is training Persian Gulf allies to “protect navigation” in the region’s troubled waterways as Washington seeks to build an alliance of friendly nations to contain Iran.

The three-week-long U.S. International Maritime Exercise (IMX), which began on Oct. 21, comes after a number of commercial vessels were attacked in the Persian Gulf from May, ratcheting up regional tensions.

Washington and other Western powers blamed the incidents on Iran.

The IMX is the second-largest maritime exercise of its kind, with 5,000 personnel, 40 vessels and 17 aircraft from 50 countries, including Japan, deployed to the strategic waterway, which separates Iran from the pro-U.S. Arab monarchies in the region.

“This is the first time we are taking part in the IMX,” the head of a Saudi naval de-mining team, Ali bin Shreidi, said aboard the Cardigan Bay, a British Royal Fleet Auxiliary landing ship dock, 40 miles (65 km) off the Bahrain coast.

The officer and his three-member team were participating in the day’s exercises dedicated to de-mining.

“We are here … to increase our capabilities and share our expertise in fighting mines, in order to protect navigation,” he said.

In June, the U.S. Navy alleged that a mine resembling Iranian weaponry was used in an attack on the Japanese-owned Kokuka Courageous tanker, which was targeted as it passed through the Gulf of Oman.

In July, Iranian Revolutionary Guards seized a British-flagged oil tanker, holding it for more than two months before releasing the vessel.

In response to the incidents, the U.S. formed a naval coalition to protect navigation in a waterway that is critical to global oil supplies.

Bahrain, which hosts the U.S. Navy’s 5th Fleet, joined the U.S.-led naval coalition in August. Saudi Arabia and the United Arab Emirates followed suit in September.

The United Kingdom and Australia are the principal Western partners to have agreed to send warships to escort commercial shipping in the Persian Gulf.

Animosity between Tehran and Washington has soared since the U.S. unilaterally abandoned a multinational deal on curbing Iran’s nuclear program last year and reimposed heavy sanctions on the Islamic republic.

Most European states have declined to participate in the naval coalition, fearful of undermining their efforts to save the nuclear accord with Iran, which was badly weakened by the U.S. withdrawal.