Two tankers carrying Saudi crude for Indian refiners have exited the Red Sea through the Bab el-Mandeb Strait by sailing “dark” after the Houthis announced a blockade on Saudi shipments, two sources with direct knowledge of the matter said, Reuters reports.

The Suezmax tanker Amazon, chartered by Indian Oil Corp, loaded 1 million barrels of oil and Aframax Rodos lifted 700,000 barrels of crude for Mangalore Refinery and Petrochemicals Ltd at Saudi Arabia’s Yanbu port around July 20 and briefly turned north towards the Suez Canal.

However, the Amazon and Rodos then switched off their Automatic Identification System transponders around July 22, preventing public tracking of their moves, and headed south to exit the Red Sea through the Bab el-Mandeb Strait, the sources said.

The tankers are the first evidence of vessels carrying Saudi crude for India going dark by turning off their transponders, which the sources said is normal practice in war zone areas, since the blockade began.

“The owner had shut the transponders on the vessels for safety purposes, and the vessels are now heading to India,” one of the sources told Reuters.