Russian President Vladimir Putin has landed in Turkey to meet his counterpart Recep Tayyip Erdogan as relations between the two states continue to thaw. The situation in Syria and de-escalation zones are expected to be the main issues on the table.

The leaders held a phone conversation on Monday in which they discussed Syria, including de-escalation zones – an initiative proposed by Russia and also brokered by Iran and Turkey – and political settlement of the crisis, according to the statement from the Kremlin.

The parties agreed to further discuss the Syria issue during Putin’s visit to Turkey on Thursday. The leaders will meet in the Presidential Complex in Ankara, Erdogan’s office said in a statement.

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Bilateral relations between Turkey and Russia will also be on the table during the meeting, the statement added.

On Tuesday, Kremlin spokesman Dmitry Peskov confirmed that the presidents had discussed the independence referendum of Iraqi Kurdistan on Monday.

The topic of S-400, Russia’s most advanced missile systems which Ankara is planning to purchase, remains hotly-discussed in the media. It’s not yet clear whether Putin and Erdogan will discuss it.

Speaking about military cooperation between Moscow and Ankara, Peskov said on Wednesday that “no one has a right to criticize it.”

Moscow-Ankara cooperation is performed “in strict accordance with international law and in no way is directed against any third country,” he said.

READ MORE: Turkey confirms deposit on Russia’s state-of-the-art S-400 missile systems

Earlier in September, Putin’s aide on military-technical cooperation, Vladimir Kozhin, said that the contract has been “agreed upon,” and Ankara added that a deposit has been paid.

The deal signals closer cooperation with Moscow and a widening rift with its NATO allies, which have criticized the deal, claiming that the Russian system may be incompatible with those of NATO.

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US Defense Secretary James Mattis said in July that S-400 anti-aircraft systems are “not going to be interoperable with NATO systems.”

Erdogan rebuffed the critics, saying Ankara had no intention of waiting for the protection of its NATO allies.

“They have gone crazy because we made a deal for S-400s,” Erdogan said.

“What do you expect? Should we wait for you? We take care of ourselves in every security point,” he added.

S-400 delivery to Turkey will start within two years, Defense Undersecretary Ismail Demir said on Monday, as cited by Anadolu news agency.

This is the second visit of the Russian leader to Turkey since the improvement in relations after downing of the Russian jet by Turkish forces in November 2015.

Relations between the two states began to thaw in June 2016 after Erdogan sent a letter to the Kremlin apologizing for the pilot’s death.

Putin visited Istanbul in October 2016 to discuss Syria, as well as stalled energy projects and bilateral trade.