By Florence Tan, Nidhi Verma, Jonathan Saul and Ahmad Ghaddar, ReutersThree oil tankers loaded with Saudi Arabian crude bound for China and India made U-turns in the Red Sea on Tuesday, following a warning from Yemen's Iran-aligned Houthi militia.Instead, they headed towards the Suez Canal rather than braving the Yemeni coast.The ships' change in direction forewarn a second potential disruption to a major shipping route that could compound the shortfall in global energy markets caused by the closure of the Strait of Hormuz.On Monday, the Houthis declared a naval blockade against Saudi Arabia, opening a potential new front in the US-Israeli war with Iran and raising the threat to global energy supplies and trade beyond the Gulf.In an email sent to shipping companies, the militant group warned against loading or discharging cargo at Saudi ports, saying it may result in their ships being targeted "in any location".The Houthis control northern Yemen, including the coast of the Bab el-Mandeb, the strait at the mouth of the Red Sea.Since the closure of the Strait of Hormuz, Saudi Arabia's Red Sea port of Yanbu has become the main alternative route for Middle East oil, allowing exports of millions of barrels per day.War risk insurance costs have already risen in the past 24 hours, with risk assessments for Saudi ports being re-evaluated, insurance industry sources said on Tuesday."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 as being at "high risk".U-turns in the Red SeaReuters reported the following details about the three oil tankers that turned around following the Houthi threats:The Xin Long Yang is a Very Large Crude Carrier (VLCC) carrying 2 million barrels of Saudi crude for China.The Rodos is a smaller tanker that loaded about 700,000 barrels of Saudi crude intended for India.The Amazon is a 'Suezmax' tanker that loaded crude on Tuesday destined for India.All three tankers switched off their transponders on Tuesday.Dynacom Tankers Management, which manages Rodos and Amazon, and Cosco Shipping, manager of Xin Long Yang, did not immediately respond to requests for comment.Another VLCC, the New Prime, which was scheduled to arrive at Yanbu later this week to load crude, has also turned back off Oman before entering the Red Sea. Its manager did not respond to a request for comment either.Three shipping sources said Yanbu port was still operating to load oil onto ships that were already in the Red Sea or arriving via Suez.Ship tracking data showed another tanker, the Olympic Luck, was still heading for Yanbu, as were several ships that were already close by.However, most tankers sailing in the Red Sea or loading at Yanbu have switched off their transponders since Tuesday.Suez detour adds weeks to voyageForcing tankers to use the Suez Canal rather than exit the Red Sea through the Bab el-Mandeb would add weeks to shipments to customers in Asia, requiring sailing through the Mediterranean and around Africa."While an actual blockade seems unlikely given the resources required for such an undertaking, a step-up in hostilities could see Houthis targeting Saudi-associated ships transiting the Bab el-Mandeb strait," ship broker Clarksons said in a note on Tuesday.In recent months, an average of 10 crude tankers have sailed through the Bab el-Mandeb daily. However, should the Houthis affect traffic through the region, "we could see a reshuffling of crude cargoes in which Yanbu volumes are increasingly directed towards Europe," Clarksons said.Fully laden VLCCs transiting the canal must first unload some of their oil, which can bypass the canal by being pumped through the SUMED pipeline to the Mediterranean.