A top US State Department official on June 26 warned Nato member Turkey that its purchase of Lockheed Martin F-35 Joint Strike Fighter jets may not go ahead if Ankara does not abandon a plan to buy S-400 missile defence systems from Russia.

The wide-ranging CAATSA sanctions law signed into law last summer by Donald Trump is aimed at punishing companies that do business with Russia’s defence industry. Turkey’s S-400 acquisition would be subject to that law, Wess Mitchell, US Assistant Secretary of State for European and Eurasian Affairs, testified in the Senate.

“We’ve also been very clear that across the board, an acquisition of S-400 will inevitably affect the prospects for Turkish military-industrial cooperation with the United States, including F-35,” Mitchell told a Foreign Relations subcommittee hearing on US relations with Europe, according to news agencies.

Ties between Washington and Ankara have been strained over issues including US policy towards Kurdish groups in Syria deemed to be a terrorist threat by Turkey, and legal cases brought against American citizens detained in Turkey, including US pastor Andrew Brunson, who is being held on terrorism charges.

Mitchell said he believed around two dozen Americans are detained in Turkey, many of them dual nationals.

He also praised Turkey as “a crucial ally and partner,” referring to its role in the campaign against the Islamic State militant and terrorist group. “We work with them very closely in intelligence and in other areas, but this [S-400 matter] has the potential to spike the punch,” he said.

On June 18, the US Senate passed a defence bill that would block the sale of F-35 fighter jets to Turkey unless Ankara scraps its S-400 deal with Russia. The provision targeting Turkey is part of the National Defense Authorization Act, which authorises over $700bn in defence spending on military programmes and weapons. The legislation was passed by 85-to-10. However, the House of Representatives has passed its own version of the bill and the two measures must be reconciled before a compromise measure can be passed and sent to Donald Trump for his signature or veto.

Mitchell, however, said the administration believes it has the legal authority to withhold the transfer of the military jets to Turkey without Congress passing the legislation.

On April 23, it was reported that the future maintenance of the UK’s F-35 fighter jets might be put at risk by Washington’s displeasure with Ankara over its planned purchase of the Russian S-400 anti-aircraft systems. Turkey builds components for the F-35 and, critically, is the US-designated country for an engine overhaul plant that will serve European F-35 customers.

The Netherlands, Italy, Denmark and Norway are other European countries buying the F-35 that may be affected by the row between the US and Turkey over the S-400.