Ankara expressed its willingness to mediate between Tehran and Washington to ease tensions over Iran’s nuclear program. In comments published Thursday, Turkish President Recep Tayyip Erdogan said he had discussed the issue of a possible mediation with Japanese Prime Minister Shinzo Abe on the sidelines of a G-20 meeting. Abe asked Erdogan whether Turkey and Japan could act together, to which Erdogan responded that he would be willing to meet with Iran’s leaders, the Associated Press reported. Erdogan spoke to Turkish journalists during a visit to China and his statements were published in the pro-government Sabah newspaper. Earlier, Iran warned that it will increase its enrichment of uranium as tensions rise a year after President Donald Trump unilaterally withdrew America from Iran’s 2015 nuclear deal with world powers. Meanwhile, Turkey-US ties are under strain over Ankaras purchase of the Russian S-400 air defence system, which is due for delivery in the coming days. Erdogan said it would be "robbery" for the United States to deny Turkey the F-35 fighter jets it has bought, according to comments published on Thursday. Washington has threatened to cancel Turkeys order of 116 F-35 fighter jets and kick it off the training and production programme, as well as wider economic sanctions, Reuters reported. "If you seek a customer, and a customer comes forward and makes payments like clockwork, how can you not give that customer their goods? This would be robbery," Erdogan was quoted as saying by Hurriyet daily. However, after meeting Trump on the sidelines of the G20, Erdogan said he had been assured there would be no sanctions, while Trump blamed the previous adminstration of Barack Obama for failing to finalize a deal for Turkey to buy the American Patriot system instead of the S-400. According to Reuters, Turkey has previously said Russia offered a better deal including joint production. In his latest comments, Erdogan said Trumps attitude was "praiseworthy."