Japan this week became the first of Iran's oil buyers to make a payment for crude imports under an interim nuclear deal, sources told Reuters on Wednesday, as the West eased a year-long stranglehold on revenues that has crippled the Iranian economy.

Tough international sanctions over the past two years have cut Iran's oil exports in half. U.S. measures imposed a year ago stopped the remaining importers of Iranian oil from transferring cash to Tehran, starving the OPEC member of its principal source of hard currency and forcing Iran to the negotiating table over its disputed nuclear program.