The US has reinforced its warnings that it would target booming Iranian oil exports after they hit a four-year high driven by demand from China.

The State Department said it was “robustly engaged” with the day-to-day business of enforcing its sanctions and cited two cases where it had targeted Chinese companies linked to the Iranian oil trade.

“We don’t preview potential sanctions actions, but we continue to monitor Iran’s oil exports and to engage Iran’s trading partners about the possibility of exposure to US sanctions,” said spokesman Ned Price.