Turkish officials have criticised a US Senate vote that could block transfers of the F-35 fighter jet to Ankara, a move that reflects increasing tensions between two Nato allies and which could ultimately hold up the sale of 100 planes worth close to $10bn.

Turkish prime minister Binali Yildirim described the move, which came just days before Sunday’s Turkish elections, as “lamentable”, against the spirit of the Nato partnership and said Ankara could look for alternatives, in comments published by the Turkish broadcaster Haberturk. 

A bipartisan clause added to the National Defence Authorisation Act — passed by the Senate late on Monday by a vote of 85 to 10 — would prohibit transfer of F-35s to Turkey until a plan is submitted to cut Ankara from participation. Turkey is one of 14 Nato members that have signed up both to buy and help produce the stealth jet, along with the UK, Norway and Israel. 

Pentagon officials and US lawmakers are concerned by Turkey’s stated intent to install a Russian air defence system, which they fear would put the secrets of the F-35 and the data it collects at risk.

The S400 Russian anti-aircraft weapon, which Turkey has said it will acquire next year, has the capacity to launch surface-to-air-missiles and relies on a powerful radar system along with autonomous detection and targeting that US officials worry could compromise the secrecy of the F-35, America’s flagship stealth fighter. 

The US has a provision to issue sanctions waivers to allies who buy military equipment from Russia but experts insist these cannot be extended to the F-35 system. 

“[Turkish president Recep Tayyip] Erdogan’s already paid a lot of money to Russia so we need to tell Erdogan that we’re not trying to create problems between him and [Russia president Vladimir] Putin, but that there is a huge concern over the F-35 specifically,” said James Jeffrey, former US ambassador to Turkey. “This is a real strategic threat to us.” 

Eric Edelman, a former senior Pentagon official who was also ambassador to Turkey, said the S400 risked compromising systems on the F-35. “The Pentagon is in a very difficult bind. Folks there had been hoping this problem would blow over and that they could talk the Turks out of it, because Turkey is too important an ally. They also didn’t want to give Erdogan the opportunity to pose as a defender of Turkish national honour against the Americans in the elections.” 

Mr Erdogan is seeking to drum up the support of nationalist voters ahead of this weekend’s elections. Analysts said the Senate vote could bolster the Turkish president, feeding into his narrative that foreign powers are conspiring to damage the country. 

US officials have said relations between the two Nato allies have in recent months reached their lowest ebb since Turkey’s invasion of northern Cyprus in 1974. 

In addition to disagreements over Syria and other issues, the Senate vote reflected US concern at the detention of American pastor Andrew Brunson, who is being held in Turkey after being charged with terrorism in the wake of a 2016 coup attempt that Ankara believes was spearheaded by a Turkish resident of the US. US officials see the detention of Mr Brunson as an unacceptable bid to secure a swap. 

But the US has also been making overtures to Turkey. Last month, US secretary of state Mike Pompeo reached preliminary agreement for a US-backed Kurdish militia, which Turkey views as a terrorist group, to leave the strategic Syrian town of Manbij at Turkey’s request. 

The diplomatic outreach risks being overtaken by Congress, however. The bill could undergo further changes in the reconciliation process with the House of Representatives, and is not expected to become law until later in the summer. 

Lockheed Martin, maker of the F-35, said it still expected to hand over the first F-35 aircraft to Turkey in a ceremonial exchange in Texas on June 21. Turkish pilots are due to be trained on the F-35 in the US, but the warplanes themselves are not scheduled to leave the country for at least a year, giving the Pentagon time to find a solution should the new law pass.

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