Oil prices swung sharply after the United States president said he had called off planned military strikes on Iran, raising hopes of progress in talks aimed at reopening the Strait of Hormuz. The vital waterway, which carries about a fifth of the world’s oil and liquefied natural gas, was effectively closed by Iran in response to earlier strikes by the United States and Israel.

Markets initially rose after the president warned Iran to move quickly toward a deal and said the ceasefire was fragile. Reports that the United States might grant a temporary waiver on sanctions for Iranian crude during negotiations later eased fears, sending prices lower.

The president said serious negotiations were under way and that regional leaders had urged him to delay military action. While expressing confidence that an acceptable agreement could be reached and that Iran would not obtain nuclear weapons, he warned that the United States military remained ready to launch a large scale assault if talks fail. Iran has not publicly responded to his latest remarks.