Russia has over the past week been delivering its advanced anti-aircraft systems, S-300, to Syria and has been using the Russian-made Antonov An-124 Ruslan for the job.

The Antonov An-124 Ruslan, also known as the Condor, is considered the largest military transport aircraft in the world, and is the second-largest plane overall, behind the Antonov An-225 Mriya. The Russian-made Mriya is the heaviest aircraft ever built and has the largest wingspan of an aircraft in service, at 88.4 meters (290 feet). With an empty weight of 314 tons, only one such aircraft was ever built.

The Ruslan weighs 192 tons empty and has a wingspan of 73.3 meters (240 feet).

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The planes, used by the Russian Air Force as well as several cargo operators, were spotted by hobbyists who track aircraft movements (also known as aircraft spotters), on the Russia-Syria route over the past several days, according to Israeli news site Ynet.

Russia said it began supplying the S-300 air-defense system to Syria on Friday, despite Israeli protests. The first Ruslan plane was spotted arriving at the Hmeimim Air Base near Latakia in Syria on Thursday evening, according to the Ynet report.

Russian Foreign Minister Sergei Lavrov announced the deliveries had started during a UN press conference. He said the anti-aircraft system “will be devoted to [ensuring] 100 percent safety and security of our men in Syria.”

Moscow’s decision to supply the systems to Syria has caused concern in Jerusalem. A senior Israeli official said Saturday that Syria’s possession of the S-300 posed a serious challenge for the Jewish state, but added that Israel was working on ways to prevent the development from becoming a major threat to the country’s security.

“The S-300 is a complex challenge for the State of Israel. We are dealing with the [decision] in different ways, not necessarily by preventing shipment [of the anti-aircraft system],” the official said.

The official added that he believes Russian President Vladimir Putin understands that while Moscow “made a move, the playing field is very large,” indicating that Israel reserved the right to protect itself and that it had the support of the United States.

Prime Minister Benjamin Netanyahu on Friday criticized Russia’s move as “irresponsible,” but said Israel was committed to continued deconfliction with Moscow in its military operations in the region.

Speaking to CNN in New York after the annual UN General Assembly, Netanyahu said that he spoke to Putin earlier this month after Syrian forces responding to an Israeli airstrike mistakenly shot down a Russian military reconnaissance plane, killing all 15 people on board.

Netanyahu said he told Putin, “Let’s continue this deconfliction, but at the same time, I told him very respectfully and very clearly that Israel will do, will continue to do what it has to do to defend itself.”

He said both sides wanted to avoid a military clash in Syria, noting that the many militaries and other groups operating in the region were making it “very crowded over there in this tiny space.

“Through this mess, we’ve been able for three years to avoid any clash between … between Russian and Israeli forces,” he said. “I think there’s a desire on both our part and Russia’s part to … avoid a clash.”

The Russian defense ministry also announced last week that it would begin jamming radars of military planes striking targets in Syria from off the coast of the Mediterranean.

Both Israel and the United States have protested the decision to supply Syria with the S-300, which could complicate ongoing Israeli efforts to prevent Iran deepening its military presence in Syria and to thwart the transfer of weapons in Syria to Hezbollah.

Israel has vowed to continue its operations.

Israel has carried out hundreds of strikes against Syrian and Iranian targets in Syria over the last several years, with fighter jets going nearly unchallenged by the country’s air defenses — though an F-16 was downed by a Syrian anti-aircraft missile in February in what the IDF later said was the result of a professional error by the pilots.

Jerusalem has vowed to prevent Lebanon-based Hezbollah or Iranian proxy militias in Syria from obtaining advanced weapons that could threaten the Jewish state and has worked to keep Iran from gaining a foothold in Syria that can be used to attack Israel.

Russia, which is a main backer of Syrian President Bashar Assad, has maintained a deconfliction hotline with Israel, allowing the Jewish state to carry out the attacks as long as it was informed beforehand.

The future of that program has been uncertain since the September 17 incident, which occurred as four Israeli fighter jets conducted an airstrike on the weapons warehouse near the coastal city of Latakia, which the IDF said was intended to provide weapons to the Hezbollah terror group and other Iranian proxies.

Moscow has accused Israel of using the IL-20 spy plane as a shield after the attack, rejecting Israel’s claims that poorly trained Syrian air defense operators are to blame for the deaths of 15 Russian servicemen aboard the aircraft.

Israel denies this charge, and insists it also notified the Russians 12 minutes before the attack — while Moscow has said it was given only a minute’s notice.

Earlier this year Defense Minister Avigdor Lieberman downplayed Israeli concerns over Russia’s purported plans to install the system in Syria.

“One thing needs to be clear: If someone shoots at our planes, we will destroy them. It doesn’t matter if it’s an S-300 or an S-700,” he said.