The US will sanction any country which buys oil from Iran after the relevant waivers expired on May 2, US Special Representative for Iran Brian Hook says.

Sanctions would be imposed "even if a country had not met its previously-negotiated purchase caps," Hook said in a statement on Thursday.

"Our firm policy is to completely zero out purchases of Iranian oil. Period."

The Wall Street Journal reported earlier on Thursday that countries like China and India, which were issued waivers in November to buy Iranian oil, could continue the purchases after May 2 until they reached a negotiated cap.