JERUSALEM — Russia recently redeployed critical military hardware and troops from Syria, according to three senior officials based in the Middle East, underscoring how its faltering invasion of Ukraine has eroded Moscow’s influence elsewhere and removing one of several obstacles to Israeli support for Ukraine.
Russia, which has been a dominant military force in Syria since 2015 and helps maintain the Syrian regime’s grip on power, still keeps a sizable presence there. But the change could herald wider shifts in the balance of power in one of the world’s most complicated conflict zones, and allow Israel, Syria’s enemy and southern neighbor, to rethink its approach to both Syria and Ukraine.
According to two senior Western diplomats and a senior Israeli defense official, speaking on condition of anonymity in order to speak more freely, Moscow recently transferred some troops and a Russian air-defense system out of Syria, removing one of the main restrictions on Israeli military actions in Syria.
The officials had varying estimates about how many troops were withdrawn — two of them said two battalions, or between 1,200 and 1,600 soldiers, while the other said it was far more. But they all agreed that the number of combat troops had been reduced.
The Israeli official also said that several Russian commanders had been redeployed from Syria to Ukraine, while Russia’s military leadership in Moscow has become less involved in day-to-day management of operations in Syria, including with military coordination with Israel.
The moves follow a reduction of Russian leverage in former Soviet countries in Central Asia, where leaders say the war in Ukraine has distracted Russia from its traditional leadership role, undermining Moscow’s aura and grip.
Since civil war broke out in Syria in 2011, President Bashar al-Assad of Syria has relied on military support from Iran and its proxies to keep rebels at bay, as well as from President Vladimir V. Putin of Russia. Russia has kept a military presence in Syria since the 1970s, but Mr. Putin significantly bolstered it in 2015 with several thousand Russian troops and aircraft, turning the tide of the Syrian war in Mr. Assad’s favor.
An enemy of both Iran and Syria, Israel regularly strikes targets in Syria to prevent Tehran from cementing a foothold close to Israel’s northeastern border. In 2018, the risk to Israeli pilots during those raids increased after Russia moved a sophisticated air-defense system, known as the S-300, to Syria.
Though control of the S-300 was never transferred to the Syrian government, the risk that it might be used against Israeli planes was a major reason Israel has rebuffed Ukrainian requests for military hardware since the Russian invasion began in February.
According to the three officials, Russia has now removed the S-300 system from Syria to bolster its flagging invasion of Ukraine — reducing Russian leverage over Israel in Syria, and altering Israeli considerations with regards to Ukraine.
Iran’s growing military support for Russia, coupled with rising Russian strikes on Ukrainian civilians, have already resurfaced a debate in Israel in recent days about whether Israel should provide arms to Ukraine. Israel produces several air-defense systems that might be useful to Ukraine in shooting down missiles and drones, including the short-range Iron Dome, which Israel uses against rockets fired from the Gaza Strip, and the longer-range Barak 8.
An Israeli cabinet minister, Nachman Shai, said on social media this week that Iran’s military assistance to Russia removed “any doubt where Israel should stand in this bloody conflict. The time has come for Ukraine to receive military aid as well, just as the USA and NATO countries provide.”
But Mr. Shai does not speak for the Israeli government. On Wednesday, the Israeli leadership stressed that while it could provide Kyiv with early-warning systems to alert Ukrainian civilians about incoming strikes, Israel would not send arms to Kyiv.
In a statement, Benny Gantz, the Israeli defense minister said: “Israel supports and stands with Ukraine, NATO and the West — this is something we have said in the past and repeat today.”
“This being said,” he added, “I would like to emphasize that Israel will not deliver weapon systems to Ukraine, due to a variety of operational considerations.”
Those considerations include Russia’s lingering military presence in Syria, including a separate air defense system, the S-400, and large air and naval bases in Western Syria. Though Russia withdrew combat troops from Syria, they have been replaced by military police officers, the senior Israeli defense official said.
Israeli officials have expressed concern that further Russian drawdowns in Syria could allow Iran to expand its influence there.
Israel also wants to avoid any disruption to practices that allows Israeli and Russian commanders to communicate with each other and avoid conflict between their forces. An encrypted telephone line was installed in 2017 to connect a Russian air base in western Syria to an Israeli Air Force command center beneath a military base in Tel Aviv.
The mechanism helps the sides avoid overlap between Russian and Israeli actions, preventing incidents like one in 2018, when Syrian forces shot down a Russian military plane that they had apparently mistaken for Israeli, killing 15 Russians.
On Monday, Dmitri A. Medvedev, the deputy head of the Russian security council and former president, warned Israel against any “reckless” supply of military aid to Ukraine. “It will destroy all interstate relations between our countries,” Mr. Medvedev wrote on social media.
Nevertheless, there are signs that Israel is providing more help to Ukraine than it did in the opening months of the war, when its assistance was limited to mainly humanitarian aid, including a field hospital.
Israel is providing Ukraine with basic intelligence about Iranian drones used by Russia, and has also offered to examine the remains of drones that crashed in Ukraine, according to senior Ukrainian and Israeli officials.
Patrick Kingsley reported from Jerusalem and Ronen Bergman from Berlin. Hwaida Saad contributed reporting from Beirut, Lebanon.