The Israeli military on Tuesday acknowledged conducting an airstrike against a Syrian weapons facility the night before and “expressed sorrow” for the deaths of 15 Russian soldiers, whose plane was shot down during the attack by Syrian air defenses.

The highly irregular move came as Moscow fumed over the incident and threatened unspecified “measures,” saying it held Israel wholly responsible.

In a statement, the Israel Defense Forces denied all responsibility for the downing of the Russian spy plane, saying that Syria, Iran and Hezbollah were the ones at fault.

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“Israel expresses sorrow for the death of the aircrew members of the Russian plane that was downed tonight due to Syrian anti-aircraft fire,” the IDF said, and noted that the Russian plane that was hit “was not within the area of the operation.”

Shortly after the military released its statement, a senior Israeli official announced that Prime Minister Benjamin Netanyahu and Russian President Vladmir Putin were slated to speak about the matter later in the day.

The Israeli strike was conducted at approximately 10 p.m. by four F-16 fighter jets, according to the Russian military.

Syrian air defenses opened fire at the incoming missiles, at the attacking aircraft and — according to Israel — at nothing in particular. The Russian Il-20 was shot down in the air battle, along with its 15-person crew.

“The Syrian anti-air batteries fired indiscriminately and, from what we understand, did not bother to ensure that no Russian planes were in the air,” the army said.

According to the IDF, the target of its Monday night strike was a Syrian military facility that manufactured “accurate and lethal weapons,” which were “about to be transferred, on behalf of Iran, to Hezbollah in Lebanon.”

The target of the Israeli strike was identified by Syria as a subsidiary of its defense ministry, known as the Organization for Technical Industries, which has suspected ties to the country’s chemical weapons and missile programs.

“These weapons were meant to attack Israel, and posed an intolerable threat against it,” the army said.

Though Israeli officials have said, generally, that military conducts operations inside Syria against Iranian and Hezbollah targets, the IDF rarely acknowledges specific airstrikes, preferring instead to adopt a formal policy of neither confirming nor denying the attacks attributed to it.

The military said its initial investigation found that its strike was completed before the Russian plane entered the area of the operation and that the reconnaissance aircraft was shot down after the Israeli fighter jets had returned to Israeli airspace.

“Israel holds the Assad regime, whose military shot down the Russian plane, fully responsible for this incident. Israel also holds Iran and the Hezbollah terror organization accountable for this unfortunate incident,” the army added.

This appeared to refute the claim made by Moscow that the Israeli pilots used the surveillance plane as cover for their attack.

Moscow had earlier said rockets were fired from a French frigate positioned in the same area of the Mediterranean.

But the French military denied any involvement, with spokesman Colonel Patrik Steiger telling AFP the frigate Auvergne “did not fire anything last night.”

Russian Defense Minister Sergei Shoigu also accused Israel of failing to inform the Russian military of its plans, which he said would have been in the “spirit” of Israeli-Russian coordination in Syria. The Russian defense ministry said Israel warned them of the impending strike “less than a minute” before it began, which left them insufficient time to clear their personnel from the area.

The Israeli and Russian militaries maintain what they call a “deconfliction mechanism,” which is meant to coordinate their activities in Syria in order to avoid incidents like this one. Until Monday night, these efforts had largely succeeded in preventing direct or indirect clashes since Russia became more deeply involved in the Syrian civil war three years ago.

The Israeli military said it had coordinated with Russia ahead of the attack, though it did not address Moscow’s specific claims about the amount of time between the notification and the airstrike itself.

The IDF also said it would “share all the relevant information with the Russian Government to review the incident and to confirm the facts in this inquiry.”

Earlier on Tuesday, the Russian Foreign Ministry summoned Israel’s ambassador for a meeting, state media reported, over what Moscow described as an Israeli “provocation.”

The summons came hours after Shoigu told his Israeli counterpart Avigdor Liberman that Moscow was holding Israel fully responsible for the downing of the aircraft and threatened possible “countermeasures,” amid fears the incident could send ties between Moscow and Jerusalem spiraling.

“We consider these provocative actions by Israel as hostile. Fifteen Russian military service members have died because of the irresponsible actions of the Israeli military. This is absolutely contrary to the spirit of the Russian-Israeli partnership,” the Russian military said in a statement, according to Russia Today, a Kremlin-linked news outlet.

A spokesperson for Israel’s Foreign Ministry refused to comment.

Russian Defense Ministry spokesman Igor Konashenkov told the state-run TASS news agency that Israel knew the reconnaissance plane was there and used it as cover to carry out the airstrike.

“By using the Russian plane as a cover the Israeli air pilots made it vulnerable to Syrian air defense fire. As a result, the Ilyushin-20, its reflective surface being far greater than that of F-16, was downed by a missile launched with the S-200 system,” Konashenkov said.

Some pilots, however, have expressed doubt about this account, saying it is unlikely that the Russian plane was used in this fashion.

“You can’t ‘hide behind a plane’ — the Russian claim is unprofessional and attempts to clear the Syrians of guilt,” said former IAF pilot and Military Intelligence chief Amos Yadlin.

In the phone call, Shoigu told Liberman that Russia “reserves the right to further countermeasures” against Israel, according to the state-run Interfax news outlet.

Shoigu added that the “actions of the Israeli Defense Ministry fail to match the spirit of Russian-Israeli partnership,” according to Interfax.

“Israel was repeatedly asked not to deliver strikes on Syria endangering Russian personnel,” Interfax quoted Shoigu as telling Liberman.

Liberman’s office confirmed that the minister had spoken with Shoigu, but said it “would not comment beyond that.”

It was not immediately clear how the downing of the Ilyushin Il-20 plane would affect the ongoing military cooperation between Israel and Russia in Syria.

A Kremlin spokesperson said the situation was being “analyzed” in light of the incident, according to Interfax.

Despite being allied with Syrian President Bashar Assad and Iran, Russia has mostly turned a blind eye to reported Israeli attacks on Syrian and Iranian facilities in the country.

The S-200 air defense system that shot down the Russian spy plane is manufactured by Russia and sold to Syria. In February, the same type of system was used to shoot down an Israeli F-16 fighter jet that was taking part in airstrikes in Syria in response to an Iranian drone that was flown into Israeli airspace from a Syrian air base earlier in the day.

Syria’s state media had reported Monday night that the state company for technical industries had been bombed, triggering the country’s air defenses.

Two people were killed in the strike and eight more were injured, according to the Britain-based Syrian Observatory for Human Rights, which said the target appeared to be an ammunition depot, part of the company’s compound. The war monitoring group said it was not clear if the depot was for Iranian or Syrian forces.

Times of Israel staff and Agencies contributed to this report.