"Part of the emphasis with our multi-domain operations that we're doing with the Army is trying to find ways that we can defeat these systems together, so that we can get in there faster and be more effective sooner," Gen. Mike "Mobile" Holmes, commander Air Combat Command, told reporters during a breakfast in Washington, D.C.
While Holmes said the S-400's capability in comparison to its S-300 long-range predecessor is an incremental upgrade at best, his concerns stem from the upgraded range of the S-400, a new mobile missile battery called an "F-35 killer" by Moscow.
"Constantly evolving defensive systems, constantly evolving offensive systems, the big things that we see are increasing range and increasing sensitivity of the sensors that are deployed with the evolving surface-to-air defenses," Holmes said.
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"Part of the main issue here is the increasing range of the new systems that are being deployed, and so it limits the range, particularly of your [fourth-generation] legacy aircraft, of how close they can get to targets until you're able to go counter these," he added.
The limitation also means that if aircraft are far back, they have less time to operate with the support of refueling tankers, and thus won't be able to operate for as long, which could stifle an operation.
"We continue to work to use all our tools to make sure that we have a way to counter them, and we do, but it requires pulling together a lot of tools to operate in an integrated way to go do it. So the concern is, it pushes you back," Holmes said.
The advanced system, known as the "Triumf," has been spotted in Syria and can carry multiple short- to very long-range missiles with a variety of sensor systems. It has a range of 400 kilometers and is effective against stealth aircraft, ballistic missiles and cruise missiles, according to the Congressional Research Service.
When asked if the Air Force is looking to leverage the Army's Long Range Precision Fires (LRPF) Missiles or if officials are looking to develop a specific hypersonic missile weapon, Holmes demurred, but said the work remains a joint effort.
"We have capabilities that we'll bring together in [intelligence, surveillance and reconnaissance] and being able to command and control things, and the different long range fires that we bring to the fight," he said.
Holmes' comments come as U.S. lawmakers are attempting to block sales of the F-35 Joint Strike Fighter to Turkey over what some officials say are about human rights violations, while others say it's over Turkey's recent S-400 purchase from Russia.
In April, NATO-ally Turkey firmed up a $2.5 billion deal for the purchase of S-400 surface-to-air missile systems. Turkey last year agreed to purchase four S-400 missile batteries over the next few years. It then finalized the deal in September.
Last week, Senate officials added an amendment to a foreign aid bill that would halt Lockheed Martin Corp's deliveries to Turkey over proposed future S-400 purchases.
The amendment came as Lockheed, the fifth-generation aircraft's manufacturer, hosted Turkish officials at its Fort Worth, Texas, facility during a ceremony celebrating the first delivery of the F-35A to the NATO ally.
"The problem is, is how do you interoperate with NATO systems with Russians; they'll never interoperate," Defense Secretary Jim Mattis told reporters last year after Turkey verbally agreed to the S-400 purchase.
"So...do they actually employ it? Do they only employ it in one area? But we'll have to take a look at it; Obviously, it's not going to be interoperable," Mattis said at the time.
-- Oriana Pawlyk can be reached at firstname.lastname@example.org. Follow her on Twitter at @oriana0214.