The United States has demanded that Turkey cancel all purchases of the Russian-made S-400 system and instead buy the US-made Patriot missile system. The delivery of the Russian defence missiles to Ankara began in July 2019 and installation and deployment is ongoing, according to reports.

US State Department spokeswoman Morgan Ortagus stated that the US continues to strongly object to Turkey purchasing and implementing Russian-made S-400 missiles, adding that it could result in sanctions on the country through the use of the Countering America's Adversaries Through Sanctions Act (CAATSA) . 

"We continue to stress at the highest levels that the S-400 transaction is the subject of ongoing CAATSA sanctions deliberations and it remains a major obstacle in the bilateral relationship and at NATO. We are confident that President Erdogan and his senior officials understand our position," Ortagus said in an emailed statement.

The need to focus on slowing the spread the novel coronavirus pandemic has purportedly delayed, but not cancelled, implementing the new S-400 defence system, according to an unnamed senior Turkish official.

"There is no going back on the decision to activate the S-400s (but) due to COVID-19 ... the plan for them to be ready in April will be delayed," the official said, cited by Reuters.

The United States claims that the S-400 missiles, delivered by Moscow to Turkey in July 2019, are incompatible with NATO defences and would jeopardise an agreement with Turkey for purchasing US fifth-generation F-35 stealth fighter jets. Instead of the S-400, the US has repeatedly demanded that Turkey purchase the US-made Patriot air defence system. 

Ankara has refused to make any concessions regarding the S-400 purchase. Earlier in March, the country's president, Recep Tayyip Erdogan, declared that the S-400 would enter service in Turkey in April.