Washington (AFP) - The United States could still block the delivery of F-35 stealth jets to Turkey if Ankara buys Russia's S-400 air defense system, a senior official said Wednesday.

US defense giant Lockheed Martin delivered an F-35 to Turkish officials in Texas last week, but Assistant Secretary of State Wess Mitchell warned the program could still be halted.

The advanced jets will remain on US soil in Arizona while their Turkish pilots are trained, giving the State Department time to intervene, the top official told a Senate hearing.

"In this program the US maintains custody of aircraft until they are transferred. That normally occurs after a lengthy training process," Mitchell told the Senate Foreign Relations committee.

"We believe that we have existing legal authorities that would allow us to withhold transfer under certain circumstances, including national security concerns," he said.

"Given that, we believe that we continue to have time and ability to assure that Turkey does not move forward on S-400 before having to take a decision on F-35," he explained.

Turkey's President Recep Tayyip Erdogan was re-elected at the weekend with increased presidential powers.

His administration is in dispute with its NATO ally Washington on a number of points, including its repeatedly stated intent to deploy Russia's S-400 anti-aircraft missile system.

This would mean Turkey's defenses are not compatible with those of its Western allies and put it in breach of US sanctions aimed at hobbling Russia's defense export sector.

US lawmakers are working on ways to punish Turkey if it buys the S-400 or if it continues to imprison two dozen US citizens on "unjust" charges under Erdogan's state of emergency.

But Mitchell said President Donald Trump's administration already has the powers its needs under existing sanctions laws.

He said the US had been clear that "an acquisition of S-400 will inevitably affect the prospects for Turkish military-industrial cooperation with the United States, including F-35s."

"We can't be any clearer in saying, both privately and publicly: a decision on S-400s will qualitatively change the US-Turkish relationship in a way that would be very difficult to repair."

Turkey has said the agreement on the S-400 system had already been reached, but Mitchell told lawmakers that Washington would judge this "when there's actual delivery of the technology."