The U.S. is beefing up its use of fighter jets around the strategic Strait of Hormuz to protect ships from Iranian seizures, a senior defense official said Friday, adding that the U.S. is increasingly concerned about the growing ties between Iran, Russia and Syria across the Middle East.

Speaking to Pentagon reporters, the official said the U.S. will send F-16 fighter jets to the Gulf region this weekend to augment the A-10 attack aircraft that have been patrolling there for more than a week. The move comes after Iran tried to seize two oil tankers near the strait last week, opening fire on one of them.

The defense official, who spoke on condition of anonymity to provide details of military operations in the region, said the F-16s will give air cover to the ships moving through the waterway and increase the military’s visibility in the area, as a deterrent to Iran.