LONDON (Bloomberg) -- Iran’s oil shipments tumbled this month after the U.S. ended sanctions waivers that allowed eight governments to buy from the Persian Gulf country. So far, not a single ship has been seen leaving Iran’s oil terminals for foreign ports in tanker tracking data compiled by Bloomberg.

China, India, South Korea and Japan were among those allowed to buy about 1 MMbpd of Iranian crude and condensate, a light form of oil extracted from gas fields, until May 2. Hopes that those exemptions might be extended were dashed, leaving buyers at risk of U.S. penalties if they continued to take Iran’s oil. They seem not to have taken that chance.

China, South Korea and Japan had already taken a cautious approach to the end of waivers, cutting their purchases of Iran’s oil in April. The long voyages from the Persian Gulf to northeast Asia, taking as much as a month, meant that oil lifted from Iranian terminals in April would not arrive until after the waivers had expired, leaving buyers at risk of reprisals.

Iran’s oil ministry, National Iranian Oil and National Iranian Tanker declined to comment.

More news from Iran: Iran hasn't cut offshore oil nor halted exports, according to IOOC

The much shorter voyages to India meant that refiners there could keep buying Iranian oil until well into April and still see it arrive before May 2. As a result, the volume of crude seen leaving Iran for India in April was the most in seven months, at 400,000 bpd.

While tanker tracking provides a valuable indication of shipments, it does have limitations. Ships’ captains can turn off the transponders that signal their vessel’s position, hiding them from view. While this tactic has been employed by Iran since November, ships have usually reappeared several days after leaving the Persian Gulf, or when passing choke points such as the Suez Canal or Singapore.

As of May 9, there were four Iranian tankers anchored off China, capable of holding a combined 5 MMbbl, with another supertanker on its way. Two more very large crude carriers, each able to haul 2 MMbbl, were observed at Indian ports waiting to discharge the last cargoes loaded in April.

Most of the rest of the Iranian tanker fleet is either heading back toward the Persian Gulf after discharging cargoes or have been observed in or near the region in the past two days.

There are some Iranian tankers that may still be hauling cargoes. No tracking signals have been received from 10 VLCCs for at least 16 days.

These could have loaded Iranian oil and left the Persian Gulf, but until they reappear it is more likely that they are being used for floating storage near the country’s loading terminals.

It is also possible that non-Iranian vessels could be picking up cargoes with their transponders turned off.