The Russian Defense Ministry announced that Russia has sent the fourth shipment of advanced air defense equipment (S-400 missile system) to Turkey, yesterday, continuing the implementation of a deal will likely lead to the imposition of US sanctions on Ankara, a member of NATO, A military source said that another Russian cargo, including 120 guided missiles, will be transported by sea to Turkey by the end of the summer.

The ministry said the fourth Russian cargo plane landed at the Mereded airbase near the capital Ankara, a day after the unloading of three large cargo aircraft of the AR-124 of the Russian air force, equipment at the base.

Washington has been trying to block the deal for months, saying Russia's S-400 air defense system is not compatible with NATO regulations, and also says that if the S-400 is deployed near US F-35 fighter jets purchased and helped by Turkey, The defenses of this stealth fighter will be undermined.

US officials earlier threatened to exclude Ankara from the F-35 if it received the S-400 system and said it would also face sanctions under a US law aimed at preventing countries from buying military hardware from Russia.

Turkey says the missile defense system is a strategic defense need to secure its southern border with Syria and Iraq. It also says the United States and Europe did not offer it a suitable alternative when it concluded the S-400 deal with Russia.

The disagreement among NATO participants is a deep division of the Western military alliance, formed after World War II to counter Moscow's military power.

Washington's response was limited yesterday by Defense Secretary Mark Esper's statement that the US position had not changed. Esper later spoke with his Turkish counterpart Khulosi Akar.

The Turkish Defense Ministry said in a statement: "Minister Akar told his US counterpart that Turkey is still under a serious air and missile threat, and that the purchase of S-400 defensive systems was not an option but a necessity."

Investors in Turkey are worried about the deal and the prospect of sanctions after a year of wrangling with Washington over the trial of a US priest in Turkey that has led to a financial crisis that has pushed the economy into recession, Reuters reported.

The Russian news agency Tass quoted an unnamed military source as saying that another Russian cargo, including 120 guided missiles, would be transported by sea to Turkey at the end of the summer.

Four senior US senators from the two main parties called on United States President Donald Trump to impose sanctions on Turkey after Ankara began acquiring Russia's air defense system.

"We urge President Trump to fully implement the sanctions as required by law," the four senators wrote in the letter.

NATO warned yesterday of "serious consequences" for Turkey's acquisition of the Russian air-to-air defense system following the arrival of the first shipment. "It is up to the allies to make a decision on the nature of Military equipment, but we are concerned about the possible adverse consequences of Turkey's decision to buy the S-400 system. "

The cooperation between Turkey and Russia came after a serious diplomatic crisis between the two countries. On November 24, 2015, two F-16 fighter jets, a Russian S-24 fighter, were dropped. Ankara confirmed that it was in Turkish airspace, Dropped in Syrian airspace.

On 27 June 2016, Russian President Recep Tayyip Erdogan sent a letter to his Russian counterpart Vladimir Putin expressing his regret over the killing of the Russian pilot, offering his condolences. The Kremlin spoke of apologies. Turkey, and then ordered Putin to lift sanctions in the field of tourism and normalization of trade relations, after the first telephone conversation with Erdogan since the crisis.

On April 3, 2018 Putin and Erdogan gave the green light to build the first nuclear reactor in Turkey. The Akoyo station in Mersin province will be built by the giant Rosatom Group. The reactor will enter the service in 2023. In September of the same year Moscow and Ankara announced an agreement on the region Demilitarized zone in the Syrian province of Idlib separates the territory under the control of opposition fighters and those controlled by government forces, which led to avoid an attack by the Syrian army on the province in northeastern Syria.

Investors in Turkey are worried about possible sanctions against Ankara.