Syria antiaircraft fire downed a Russian military plane after an Israeli attack on Syrian positions, killing 15 people on board, Moscow said.
Russian state media said Syrian missiles shot the maritime patrol aircraft down amid an attack by Israeli jets in the Latakia region of northwest Syria.
Moscow blamed Israel for putting its aircraft in the line of fire and said it had only a minute's notice of the strike.
"As a result of the irresponsible actions of the Israeli military, 15 Russian servicemen were killed, which is absolutely not in keeping with the spirit of Russian-Israeli partnership," said Maj. Gen. Igor Konashenkov, spokesperson for the Russian military, according to RIA-Novosti.
In a rare acknowledgment of its military activity in the region, Israel expressed "sorrow" for the loss of Russian life, but blamed the Syrian regime of Bashar al-Assad. The Israeli Defense Forces said it had followed well-established protocols in communicating with Moscow, which are intended to prevent such incidents.
"Israel holds the Assad regime, whose military shot down the Russian plane, fully responsible for this incident," the IDF said in a statement, adding that Iran and Hezbollah were also accountable.
According to the statement, Syrian antiaircraft batteries "fired indiscriminately and from what we understand did not bother to ensure no Russian planes were in the air."
The incident presents Moscow with a diplomatic conundrum, as the country has a strong relationship with both Israel and the Syrian regime.
On Tuesday Russian President Vladimir Putin called the incident a tragedy and said that Russia will concentrate on taking additional security measures to "protect our servicemen and our assets in Syria."
"Here it's more like a chain of tragic accidental circumstances, because Israel didn't shoot our plane down. But there's no doubt, no doubt at all, that we will have to take a serious look at what in fact happened, and our view of this tragedy is set out in the statement issued by the Russian Federation Ministry of Defense, which has been fully agreed with me," Putin said.
In a phone conversation with Putin, Israeli Prime Minister Benjamin Netanyahu reaffirmed "the importance of the continuation of security coordination between Israel and Russia, that has succeeded in preventing many losses on both sides in the past three years," according to a statement released by the Prime Minister's Office.
Netanyahu promised that Israel would transfer all relevant information about the incident to Russia and proposed sending the Commander of the Israeli Air Force to Moscow to personally hand over the information.
Earlier in the day Israeli Defense Minister Avigdor Liberman talked by phone with his Russian counterpart, Army Gen. Sergei Shoigu, both sides said. According to a readout of the call from the Russian Ministry of Defense, Shoigu described the actions of the Israeli air force as irresponsible and told Liberman that fault "rests entirely with the Israeli side."
"We reserve the right for further reciprocal steps," Shoigu said, according to the readout.
At a press conference Tuesday, US President Donald Trump said it was his understanding that Syria was responsible for the incident.
"That's not a good situation," Trump said at a press conference, noting reports were still preliminary. He called it a "very sad thing."
Trump defended his strategy in Syria, saying "we have done a tremendous job" battling ISIS in the war-torn nation.
Anti-aircraft system 'sold to Syria by Russians'
The target in Syria's Latakia coastal region that Israeli warplanes struck Monday was believed to be a facility that Iran was using to house sensitive military equipment that could be assembled into weapons, and then transferred to Lebanese Hezbollah, according to two US defense officials.
The officials would not detail the intelligence that led the United States to this conclusion. But they pointed out a major concern of both Israel and the United States: that Lebanese Hezbollah is gaining access to Iranian missiles that are increasingly precise and longer range in order to attack Israel.
US and Israel surveillance routinely monitors Iranian air and ground shipments into Syria. But the identification of a site focusing specifically on assembly of weapons underscores an effort by Tehran to continue to try to hide the capabilities it is trying to ship to Lebanese Hezbollah.
The officials could not say if this is the first time a facility associated with assembly of weapons has been struck by the Israelis. But one of the officials said it's the US understanding that Israel has not struck in this area for nearly five years.
Earlier Tuesday, Israel hinted at the type of target it struck. The IDF wrote on Twitter that jets "targeted a facility of the Syrian Armed Forces from which systems to manufacture accurate and lethal weapons were about to be transferred on behalf of Iran to Hezbollah in Lebanon."
"These weapons were meant to attack Israel, and posed an intolerable threat against it," the IDF said.
The IDF alleges that by the time Syrian antiaircraft missiles had been launched, Israeli warplanes responsible for the overnight operation were already back within Israeli airspace.
The IDF also says that the Russian plane was not "within the area of operation" while the Israeli strike on Latakia was underway. Russia bases much of its Syrian military presence in Latakia.
The Russian military said Israel notified the Russian side about the planned operation only a minute in advance, and that Israeli controllers would have seen the Russian plane, which was coming in to land, RIA reported.
Defense Ministry spokesman Igor Yevgenyevich Konashenkov claimed the Israelis used the plane as cover, effectively putting it in the cross hairs of the Syrian air defenses, according to RIA.
The Russian aircraft was shot down by an antiaircraft system sold by Moscow to the Syrians several years ago, a US official with knowledge of the incident told CNN Monday. The Syrian air defense network in western Syria is very densely populated with antiaircraft missile and radar systems.
In February, the two-man crew of an Israeli F-16 ejected from their aircraft when a missile exploded near them, damaging their plane as they finished conducting a mission against Syrian forces.
An Israeli defense official told CNN earlier this month that Israel has struck Syria 200 times in the past 18 months to prevent the deployment of Iranian weapons in the region.
Demilitarized zone in neighboring Idlib
The incident occurred on the same day that Russia announced a joint agreement with Turkey to create a demilitarized zone in Syria's Idlib province, which neighbors Latakia, potentially thwarting a large-scale military operation and impending humanitarian disaster in the country's last rebel stronghold.
Speaking alongside Russian President Vladimir Putin at talks in Sochi on Monday, Turkish President Recep Tayyip Erdogan said the creation of a 15- to 20-kilometer (approximately nine to 12 miles) demilitarized zone will prevent a "humanitarian crisis" in the northwestern province.
All heavy military equipment tanks, ground-to-air missiles and mortars of all the opposition groups will be removed by October 10, the leaders said. The zone, which will be patrolled by Turkish and Russian military units, will become operational from October 15.
Erdogan described the agreement as a "solution" to the issues in the region.
Speaking Tuesday, Kremlin press secretary Dmitry Peskov said that the downing of the Russian aircraft would not affect the Sochi agreement "in any way," adding that "this is an important, breakthrough agreement."
In recent weeks, Syrian and Russian planes have conducted scores of airstrikes in Idlib in the run-up to an anticipated offensive by Russian-backed Syrian forces to retake the last part of the country under armed opposition.
Last week, UN officials said that more than 30,000 people fled the province in anticipation of the government offensive.