The US is beefing up its use of fighter jets around the strategic Strait of Hormuz to protect ships from Iranian seizures, a senior defence official said on Friday.
Washington is increasingly concerned about the growing ties between Iran, Russia and Syria across the Middle East, the official added.
Speaking to Pentagon reporters, the official said the US will send F-16 fighter jets to the Gulf this weekend to assist the A-10 attack aircraft that have been patrolling for more than a week.
The move comes after Iran tried to seize two oil tankers near the strait last week, firing on one of them.
The defence official, who spoke on condition of anonymity, 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.
The US Navy said in both instances the Iranian naval vessels backed off when the USS McFaul, a guided-missile destroyer, arrived on the scene.
In addition, the defence official said the US is considering a number of military options to address increasing Russian aggression in the skies over Syria, which complicated efforts to strike an ISIS leader last weekend.
The official declined to provide further detail, but said the US will not cede any territory and will continue to fly in the western part of the country on anti-ISIS missions.
The Russian military activity, which has increased in frequency and aggression since March, stems from growing co-operation and co-ordination between Moscow, Tehran and the Syrian government to try to pressure the US to leave Syria, the official said.
The official said Russia is beholden to Iran for its support in the war in Ukraine, and Tehran wants the US out of Syria so it can more easily move lethal aid to Lebanese Hezbollah and threaten Israel.
The US has seen more co-operation, collaboration, planning and intelligence sharing, largely between mid-level Russian and Iranian Quds force leaders in Syria, to pressure the US to remove troops from Syria, the official added.
There are about 900 US forces in the country, and others move in and out to conduct missions targeting ISIS militants.
The US does not believe Russian aircraft plan to drop bombs on American troops or shoot down manned aircraft.
But there are concerns that Russian pilots will knock a Reaper drone out of the sky and that Moscow believes that type of action would not get a strong US military response, the official said.
US and Russian military commanders routinely communicate over a deconfliction phone line that has been in place for several years to avoid unintended clashes in Syria, where both sides have troops on the ground and in the air.
There are often many calls a day, and at times result in angry threats as commanders argue over a continuing operation, said the US official.
Describing a conversation, the official said the Russians will often declare an area of space a restricted operating zone and say they are doing military exercises there.
The US sees no exercises, and tells Russia that American forces are on a counter-terror mission against ISI and plan to fly in that area. The Russians then say they can't guarantee US aircraft safety if they go there.
And once the mission begins, and the aircraft move into the zone, “it sometimes gets very heated", said the official, as both sides loudly protest and reject the other's assertions.
