The United States has been relentlessly trying to talk Turkey out of buying Russian anti-aircraft missile systems, but Ankara has so far been determined to go ahead with the purchase, even though it could jeopardise the Turkish-US F-35 programme and result in Washington slapping sanctions on the country.

Turkish President Recep Tayyip Erdogan has brushed off speculations about possible sanctions the US may impose against Ankara over the purchase of Russian S-400 air defence systems. Speaking at a conference in Ankara before leaving for a G20 summit in Japan, Erdogan said that judging by a phone call with Donald Trump it didn't seem that the US would introduce sanctions.

This comes as Acting US Defence Secretary Mark Esper has warned Turkey that the long-awaited purchase of Russia's S-400 air defence systems would harm Turkey's ambitions to acquire F-35 fighters as well as entail economic ramifications due to the sanctions that will likely follow, Reuters reports, citing an unnamed senior US defence official.

"The secretary was very firm, once again, that Turkey will not have both the S-400 and the F-35. And if they accept the S-400 they should accept ramifications not only to the F-35 programme but also to their economic situation", the official was quoted as saying after American and Turkish defence ministers met on the margins of a Brussels-hosted NATO summit on Wednesday.

Under a $2.5 billion contract signed in late 2017, Moscow is set to start delivering four batteries of the S-400 air defence systems to Ankara next month. The S-400s are widely considered to be one of the most advanced systems of their kind in the world.

The deal has raised concerns in both the Trump administration and NATO. They claimed that the S-400s - which are designed to track and shoot down aircraft like the F-35 - could be used to collect data on the much-touted latest-generation fighter jets.

Washington has threatened to hit Ankara with sanctions under CAATSA (Countering America's Adversaries Through Sanctions Act) and shut the door on the expected delivery of 100 F-35 jets.

The US has already halted shipments of F-35 components to Turkey and training sessions for Turkish F-35 pilots and reportedly gave the country until the end of July to backtrack on the S-400 deal if it wants to buy the F-35s.

Turkey, however, has defended its upcoming acquisition of Russian-made weapons as its sovereign right and insisted that it would not compromise the stealthy US aircraft.

Ankara said that NATO capabilities can protect only a third of its airspace, and that the missile deal is simply meant to meet its defence needs. Speaking to reporters in Ankara last week, Turkish President Erdogan described the purchase of S-400s as a "done deal" and urged restraint from the United States. He also vowed to challenge the United States in international court if Washington stands in the way of the deal.

Ismail Demir, Turkish Undersecretary for Defence Industries, has said that the United States is not able to remove it from the F-35 programme.

"This isn't part of the agreement, this isn't something you can just say 'I exclude you' about. The F-35 project is a partnership and nowhere in the agreement does it allow a unilateral removal of one country", he told reporters last week.