United States President, Donald Trump says Wednesday’s meeting between him and Turkish President, Recep Tayyip Erdogan at the White House failed to resolve an issue which has badly strained relations between the two NATO allies.

Both men ended the visit without achieving an agreement on Turkey’s decision earlier this year to accept delivery of a Russian air defence system which poses such a threat that the US suspended Turkish participation in the multinational F-35 fighter jet program.

Erdogan told reporters that he might be persuaded to use the US-made Patriot defence system as well as the Russian S-400 while Trump said they would agree to keep working on the issue. 

“The acquisition of the S-400 creates some very serious challenges for us. Hopefully we’ll be able to resolve that situation,” Trump said.

The dispute over the competing air defense systems is a major component of the tension between the two countries. Turkey has been criticized for repression of political opponents, journalists and others.

Turkey has also come under fire on Capitol Hill for its incursion into Syria last month to attack the Kurdish forces who fought with the US against the Islamic State. 

Turkey is angered at the US for supporting the Kurdish forces which it views as a threat and for refusing to extradite a Muslim cleric it accuses of fomenting a 2016 coup attempt against Erdogan. 

Despite the differences, Trump said he believes the two sides can substantially increase trade, which he said amounted to about US$24 billion in 2017.

Erdogan used the meeting as a chance to defend his military offensive against US-backed Kurdish forces in northeastern Syria, some of whom have links to the separatists who have waged a violent campaign in Turkey for decades.