Omani mediators said it was a small technical issue, Iran said new negotiations would soon be announced, and Washington acted as if nothing had been scheduled or canceled without explanation.

Iranian oil flows to China via shadow tankers were already hard to track, but now it's even tougher as the US loses some of its leverage due to the tariff war.The latest spike in US-Iran tensions has cast fresh light on one of the thorniest issues in Washington's efforts to rein in Tehran: Iran's oil exports to China -- a vital economic lifeline -- and the mounting difficulties the US faces in halting them.