Three oil tankers carrying Saudi crude to China and India made U-turns in the Red Sea on Tuesday following a threatened attack from Yemen’s Iran-aligned Houthi militia.

The tankers, heading towards the Suez Canal, changed course and turned back, according to reports.

The Xin Long Yang, Rodos and Amazon tankers’ U-turns suggest a potential second disruption to a major shipping route, which could compound the shortfall in global energy markets caused by the closure of the Strait of Hormuz.

The Houthis on Monday declared a naval blockade against Saudi Arabia, opening a potential new front in the U.S.-Israeli war with Iran and raising concerns about global energy supplies and trade beyond the Gulf.

The group warned vessels not to load or discharge cargo at Saudi Arabian ports, saying such activities could result in the vessels being targeted “in any location”.

The Houthis control northern Yemen, including parts of the coast near the Bab el-Mandeb, the strait at the mouth of the Red Sea.

“Vessels calling at Saudi Arabian ports are advised to reconsider transiting the Red Sea and to consider implementing enhanced mitigation measures,” British maritime security company Ambrey said, adding that it assessed vessels calling at Saudi Arabian ports “were at high risk”.