The pressure being applied to Nato member Turkey not to go ahead with its planned purchase of Russia’s S-400 missile defence system was cranked up even further on March 21 as the US’s top military officer described the situation as “tough”.

“It’s a tough issue,” General Joseph Dunford, chairman of the US Joint Chiefs of Staff, was quoted as saying by Reuters after he was asked about reports that Washington could pause preparations to deliver F-35 jets to Turkey as a response to Ankara’s refusal to cancel its S-400 order. Some US officials have lately started referring to the prospect of Turkey simultaneously operating the S-400 and F-35 military hardware as a “national security issue”.

Dunford was cited as telling a security forum in Washington: “Both the executive branch of our government and legislative branch of our government are going to have a hard time reconciling the presence of the S-400 and the most advanced fighter aircraft that we have, the F-35. We’re hopeful that we can find a way through this, but it is a tough issue.”

“The S-400 is a computer. The F-35 is a computer. You don’t hook your computer to your adversary’s computer and that’s basically what we would be doing,” Katie Wheelbarger, US acting assistant secretary of defence for international security affairs, told the news agency on March 20.

There is some prospect that Turkish President Recep Tayyip Erdogan might be prepared to compromise on the issue after the March 31 local elections in Turkey have taken place. After the poll, Turkey will not be due to hold another election for around four years.