Yemen’s government launched a military operation on Sunday to clear naval mines planted by the Iran-backed Houthi rebels in and near the strategic Bab al-Mandeb strait, Al Arabiya reported. The mines are “believed to have been sourced from Iran” and “threaten fisherman and residents of island near the coasts of Midi [in northwestern Yemen]” as well as “international navigation in the Bab al-Mandeb strait.”

The Associated Press reported last month that the Houthis had planted a naval mine in the strait, which killed two Yemeni sailors and wounded eight others.

The United States Office of Naval Intelligence warned in February that Houthi “naval mining threatens commercial ships traveling … near Mokha port and the Bab al Mandab Strait,” the American Enterprise Institute’s Critical Threats Project reported. An estimated 4.7 million barrels of oil pass daily through the strait, which links the Gulf of Aden to the Red Sea.

Iran and its proxies have significantly stepped up their aggressive naval behavior in recent years. Gen. Joseph Votel, head of the U.S. Central Command, accused Iran in October of orchestrating several attacks against U.S. vessels in the area that month.

The Islamic Republic is sending military advisors and advanced weapons, including drones, to the Houthis to sway the outcome of a civil war that could tip the balance of power in the Middle East, regional and Western sources told Reuters last month. The weapons are being sent via ship, either directly from Iran or indirectly via Somalia, and arrive along Yemen’s extensive coastline in typical fishing boats, which makes them harder for anti-smuggling authorities to spot.

The Houthis’ slogan is “God is great, death to America, death to Israel, a curse upon the Jews, victory for Islam.” They seized control of the Yemeni government in 2015, prompting a military intervention by a Saudi-led coalition of Arab countries. After the 2014 capture of Yemen’s capital Sana’a, Iranian parliamentarian Ali Reza Zakani, who is close to Supreme Leader Ali Khamenei, boasted that Iran now controlled four Arab capitals, the other three being Damascus, Baghdad, and Beirut.