An Iranian-flagged tanker is reloading oil confiscated in April by the United States after Greek authorities approved the release of the cargo. 

Iran's embassy in Athens and sources familiar with the matter said on Friday, about two months after Iran's Ports and Maritime Organization (PMO) said the Lana tanker is no longer impounded. The US has sanctioned Iranian oil exports since 2018.

"The operation to transfer Iran's stolen oil to the Lana ... is underway in Greek waters, and the ship will soon depart for our country with a full shipment of oil," Iran's embassy in Greece said on its Twitter account.

"The transfer has started," a source with knowledge of the matter said, while a second source said the process started on Friday and was expected to be completed in days.

Tehran had previously warned of "punitive action" against Athens over the case, which has strained relations between the two countries after the United States hired a ship to transfer the oil from the Lana tanker, when it was anchored off Greece. The US State Department said in July it was "respectful that this case went through the Greek judicial process".

The ship, previously called Pegas and renamed Lana in March, had reported an engine problem in April. It was headed to the southern Peloponnese peninsula to offload its cargo on to another tanker but rough seas forced it to moor just off Karystos where it was seized. The removal of oil from the Iranian tanker prompted Iranian forces in May to seize two Greek tankers in the Persian Gulf.