A support vessel maneuvers near the crude oil tanker 'Devon' as it sails through the Persian Gulf towards Kharq Island oil terminal to transport crude oil to export markets in Bandar Abbas, Iran, on Mar. 23, 2018. Ali Mohammadi | Bloomberg | Getty Images

Tanker-tracking firms believe Iran is once again shipping crude oil to Syria, resuming the illicit trade as tensions with Washington rise and the Islamic Republic faces increasing international isolation. An Iranian delivery of approximately one million barrels of crude was made into the Syrian port of Baniyas during the first week of May, according to TankerTrackers.com and ClipperData, two groups that follows oil vessels. This would be the first Iranian oil delivery to Syria since the end of 2018, according to Samir Madani, founder of TankerTrackers. The suspected delivery comes one year after the U.S. unilaterally pulled out of an international nuclear agreement with Iran and just one week after the Trump administration tightened energy sanctions in an effort to push Iranian crude exports to zero. It also follows the deployment of a U.S. carrier strike group and bomber task force to the Middle East earlier this week.

Iran's leadership in Tehran responded on Wednesday by announcing it will stop complying with key parts of the nuclear accord. The same day, Washington slapped new sanctions on Iranian metal exports. Analysts have widely predicted that Iran will step up efforts to smuggle oil into Syria and neighboring Iraq as the U.S. makes it more difficult for Tehran to ship oil to its few remaining customers, including China, India and Turkey. Madani, the TankerTrackers founder, has tracked Iranian vessels for nearly three years, both through data collected by the U.S. Coast Guard and what he calls, "eyes in the sky," essentially visual data collected by a network of satellites. This allows the group to track tankers that may turn off their transponders — a common move for those selling oil off the markets. The most recent moves by an Iranian tanker, previously known as True Ocean, caught the attention of both tracking companies when the vessel made a series of unusual moves. The Suezmax tanker in question, now traveling as the Masal, is on the U.S Treasury's list of vessels that has been used to move oil to Syria in the past.

Based on the U.S. Coast Guard's AIS data, the Masal left Iran in March heading towards the eastern ports of Turkey. Madani points out that this was the first sign that things were different. Typically Iranian tankers head to the western ports where Turkish refineries are based. Once in Turkey, the vessel is said to have anchored for three weeks before leaving the port of Iskenderun, still carrying a million barrels of oil. This tipped off TankerTrackers to keep a close eye on the Masal. However, the tanker eventually went offline, switching off its transponder near the Syrian shoreline. According to TankerTrackers and ClipperData, the Masal didn't come back online until May 7, at which point it had unloaded its cargo near the port of Baniyas where Syria's largest refinery is located. It then began making its way back to Iran. The Foreign Ministry of Iran did not immediately return a request for clarification on the suspected delivery to Syria. ClipperData says the Masal is the second ship carrying Iranian oil that has been caught going dark in the Mediterranean since April.