"It's certainly disappointing," Defense Secretary nominee Mark Esper told his Senate confirmation hearing.

"Turkey has been a long-standing and very capable NATO ally, but their decision on the S-400 is the wrong one and it's disappointing," he said.

Esper said he would abide by the decision last year to drop Turkey from NATO's F-35 fighter jet development program due to Ankara's decision to buy the S-400 system, the delivery of which began on Friday.

Esper said he had already told Turkish Defense Minister Hulusi Akar that his policy is, "You can either have the S-400 or the F-35. You cannot have both."

"Acquisition of the S-400 fundamentally undermines the capability of the F-35 and our ability to maintain that overmatch in the skies going forward," Esper told the Senate panel.

Despite warning Ankara multiple times over the past two years that it could face sanctions and other retaliatory action for acquiring the Russian system, Washington officials have been silent since the deliveries began Friday.

It was not clear if new sanctions, demanded by the US Congress, would be enacted.

But in May the Pentagon said it was preparing to cut Turkey out of the network of manufacturers supplying parts to the F-35.

A number of Turkish manufacturers are making parts and equipment for the F-35, including internally carried Stand-off Missiles, airframe assemblies and wiring, leaving the NATO program partially dependent on them.

On Monday Turkish President Recep Tayyip Erdogan hailed the delivery of the first S-400 equipment and said the next step was to jointly produce the system.

"We have begun to receive our S-400s. Some said, 'they cannot buy them'... God willing the final part of this (delivery) will be in April 2020," Erdogan told a crowd of several thousand in Ankara.