CNN’s Nic Robertson reports from Islamabad, Pakistan, where a second round of US-Iranian peace talks are scheduled for next week.

Two gunboats from Iran’s Revolutionary Guard opened fire on a tanker attempting to cross the Strait of Hormuz on Saturday, shortly after Iran said it had reimposed restrictions on the vital waterway.

The incident took place 20 nautical miles northeast of Oman, with the tanker's captain stating that two gunboats opened fire without issuing a radio challenge.