Second delivery of S-400 system will take around one month The two also spoke about working together to ease tensions in Sryia"s Idlib province MOSCOW: Russia delivered another battery of Russian S-400 missile defences on Tuesday President Vladimir Putin said. "By the way, another delivery was made this morning," Putin told Turkish President Tayyip Erdogan during a visit to Russia. Turkey"s Defense Ministry said that the delivery of a second battery of S-400 defence system had started as of Tuesday and that it would take around one month. Erdogan and Putin also opened a major Russian air show on Tuesday, where Erdogan was Putin"s guest of honor. The two leaders attended the opening of the MAKS aviation show outside Moscow and even got to peek inside the cockpit of Russia"s newest fighter jet, the Su-57, which was unveiled at the show. Turkey"s military ties with Russia have been a cause of concern for NATO and the US after Ankara went ahead with its purchase of the S-400 missile defense system. The US says the Russian system is incompatible with NATO and poses a threat to the US-led F-35 fighter jet program. It suspended Turkey"s participation in the F-35 program and has also threatened sanctions against Ankara. The two also spoke about working together to ease tensions in Sryia"s Idlib province. "The situation in the Idlib de-escalation zone is of serious concern to us and our Turkish partners," Putin said at a joint press conference. He sympathised with Turkey, saying: "We understand Turkey"s concern regarding the security of its southern border. We believe that these are legitimate interests." Putin said the two leaders had agreed "additional joint steps" to "normalise" the situation in Idlib, but did not provide details.