Some 20 rockets were fired at Israeli military bases by Iranian forces from southern Syria just after midnight on Thursday, sparking the largest ever direct clash between Jerusalem and Tehran, with Israeli jets targeting numerous Iranian-controlled sites across Syria.

The Israeli army said the initial missile barrage was carried out by members of the Iranian Revolutionary Guard Corps’ Quds Forces. This appeared to be the first time that Israel attributed an attack directly to Iran, which generally operates through proxies.

Some of the incoming missiles were intercepted by the Iron Dome defense system, the army said. There were no reports of Israeli casualties in the attack. An IDF spokesperson said damage was caused to Israeli military bases, but that it was “limited.”

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In response, Israel launched an extensive retaliatory campaign, striking suspected Iranian bases throughout Syria for hours following the initial Iranian bombardment, an Israeli military spokesperson said, warning Syrian dictator Bashar Assad not to get involved.

“The Israel Defense Forces is taking action at this moment against Iranian targets in Syria. Any Syrian involvement against this move will be met with the utmost severity,” wrote Avichay Adraee, the Israeli military’s Arabic-language spokesperson, on Twitter.

According to Arabic media reports, the Israel Defense Forces struck numerous targets across Syria, including weapons depots and Assad regime radar and air defense systems.

The Israeli military would not immediately comment on its specific targets.

Syria’s state news agency, after initially reporting that the country’s air defenses were intercepting dozens of “hostile Israeli missiles,” later said Israeli jets hit military bases, as well as an arms depot and military radar, without specifying the locations.

Syrian rebels said these strikes targeted three airfields: the Shayrat air base, which was targeted by the United States last year for its role in an alleged chemical attack in the Syrian town of Khan Shaykhun; the Tha’lah air base, in southwest Syria, which has been tied to Hezbollah; and the Mezzeh military air field outside Damascus, which is reportedly home to Assad’s elite republican guard.

A large Israeli bombing raid was reported near the northwestern Syrian town of Qusayr near the Lebanese border, a known Hezbollah stronghold.

In the days and weeks before the Iranian barrage, defense officials repeatedly warned that Israel would respond aggressively to any attack from Syria.

On Wednesday, Prime Minister Benjamin Netanyahu met with Russian President Vladmir Putin in Moscow about Iran’s “explicit goal of attacking the State of Israel as part of their strategy to destroy the State of Israel,” he said. Netanyahu told reporters after the meeting that Putin was receptive to Israel’s demand that it be allowed to operate freely in Syria’s skies in order to defend itself.

The late-night Iranian rocket barrage and Israeli counterattack appeared to be the largest exchange in Syria since the 1973 Yom Kippur War.

In light of the aerial battles, numerous cities and towns in northern Israel decided to open their public bomb shelters, though the army did not require all of them to do so. In one case, residents of the northern Israeli town of Metula, along the Lebanese border, were instructed to take shelter after a loud explosion was heard in the area. They were later cleared to leave as no signs of impact were found.

Shortly before 3 a.m., a loud blast was also heard in the northern city of Safed, prompting its mayor to release a statement to residents reassuring them that the explosion was “not a missile strike or anything else, but rather an IDF launch from our area.”

Residents of central Israel reported hearing fighter jets flying overhead.

Tehran has repeatedly vowed revenge after the T-4 army base in Syria was struck in an air raid — widely attributed to Israel — on April 9, killing at least seven members of the IRGC, including a senior officer responsible for the group’s drone program. (Iran used the T-4 base to launch an attack drone carrying explosives into Israel in February; the drone was shot down.)

Iran apparently attempted to exact that revenge at 12:10 a.m. on Thursday, with its bombardment on Israeli military bases on the Golan Heights.

Sirens blared across the Golan Heights throughout the exchange, sending residents into bomb shelters. The IDF Home Front Command called on residents to adhere to security instructions as needed. Residents of the Golan Heights were told they could leave the bomb shelters around 2 a.m., but were instructed to remain near the fortified areas until further notice.

The pro-Syrian government Al-Mayadeen TV said more than 50 missiles — not 20, as the IDF said — had been fired from Syria toward Israeli forces on the Golan Heights. A Syrian parliamentarian claimed on Twitter that Damascus, not Tehran, had launched the attack.

Immediately following the barrage, Syrian state media reported that Israeli artillery fire targeted a military post near the city of Baath in the Quneitra border region, where Syrian regime forces were stationed.

In the hours that followed, this Israeli retaliation expanded to include more artillery strikes and aerial bombings, according to Syrian reports.

Following the initial exchange, IDF spokesperson Lt. Col. Jonathan Conricus said the army was still sorting out many of the specific details related to the missile attack, such as which army bases were targeted, what types of projectiles were used, how many rockets were fired, from where they were launched, and how many were intercepted.

A video shared on social media shortly after midnight on Thursday appeared to show the barrage of missiles apparently being fired by a multiple launch rocket systems, or MLRS, from Syria toward Israel.

Residents of both the Israeli and Syrian Golan Heights reported hearing loud, repeated explosions.

The attack came a day after the military called on local governments on the Golan Heights to open bomb shelters, in light of “abnormal” activities by Iranian forces in Syria. The barrage also followed US President Donald Trump’s announcement on Tuesday night that he was pulling the United States out of the Iran nuclear deal and reimposing sanctions on Tehran.

On Tuesday, eight Iranians were among 15 foreign pro-regime fighters killed in a suspected Israeli strike in Syria on a weapons depot of Iran’s elite Revolutionary Guards, a monitor said. The raid struck the area of Kisweh south of Damascus late Tuesday, the Britain-based Syrian Observatory for Human Rights monitoring group said on Wednesday.

Syria’s official news agency SANA said on Tuesday the army had intercepted two Israeli missiles fired toward Kisweh, with state television broadcasting images of fires in the nearby area.

Since the start of Syria’s civil war in 2011, Israel has repeatedly targeted positions of the Syrian army and the Lebanese Hezbollah terror group backing it inside the country.

On April 29, missile strikes — “probably Israeli” — fired on regime military positions near the cities of Hama and Aleppo in northern Syria killed at least 26 mostly Iranian fighters, according to the Observatory.

On April 9, missiles targeted the T-4 air base in the central province of Homs, killing up to 14 fighters, including seven Iranians, two days after an alleged chemical attack carried out by the Syrian regime.

Russia, Iran and Syria accused Israel of carrying out the strike.

In light of both the general tension and specific intelligence, the IDF went on high alert this week, deploying additional missile defense batteries in northern Israel. “There is high preparedness of IDF troops for an attack,” the army said on Tuesday.

A number of army reservists were called upon Tuesday night, the army said. An IDF spokesperson would not elaborate on which units they came from, but media reports indicated they served in air defense, intelligence and Home Front Command units.

Earlier on Tuesday, the US embassy in Israel also prohibited American government employees from visiting the Golan Heights without approval in light of the security situation on the border.

“The IDF is ready and prepared for a variety of scenarios and warns that any action against Israel will be answered with a fierce retaliation,” the army said.

On Sunday night, Israeli defense officials warned that Iran was planning to retaliate for recent deadly airstrikes in Syria, which have been attributed to the Jewish state, by having its proxies fire missiles at military targets in northern Israel sometime in the near future.

Security forces were also preparing for the possibility of attempted infiltrations of military bases and communities in the north, Hadashot TV news reported on Monday.

Agencies contributed to this report.