Three American officials on Tuesday told NBC news that Israel, using F-15 fighter jets, conducted the airstrike on an allegedly Iranian-controlled military base in northern Syria this week, in the latest bout of the increasingly public fight between Tehran and Jerusalem.

The unnamed US officials said Israel appears to be preparing for active conflict and is seeking American assistance, noting the recent visits to the US by Defense Minister Avigdor Liberman and trips to Israel by senior American officials.

According to NBC, the target of the Sunday night strike was an incoming weapons shipment, including surface-to-air missiles, which were freshly delivered from Iran.

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Casualty counts from the attack have varied, but most put the death toll at between 16 and 38, including many Iranians. Iran denied that any of its soldiers were killed in the attacks and that any of its bases in Syria were targeted.

According to NBC, one unnamed senior US official said the Iran-Israel conflict is the most likely in the world to devolve into open clashes.

“On the list of the potentials for most likely live hostility around the world, the battle between Israel and Iran in Syria is at the top of the list right now,” the official said.

The base hit in the Sunday night airstrike was located south of the city of Hama in northwestern Syria and belonged to the Syrian army’s 47th Brigade, but has reportedly been used as a headquarters for Iranian troops for several years.

فيديو للإنفجارات في ريف حماه ..نسأل الله السلامة للرجال هناك ..????الشفاء العاجل للجرحى ..حتى الآن لم يصرح مصدر مسؤول عن ماهية الإنفجارات ..#محردة_الآن Posted by ‎محردة الآن‎ on Sunday, 29 April 2018

American and Israeli officials have been monitoring as Iran has increased the number of transport planes that it sends from its Mehrabad Airport in Tehran to Syria. The US and Israeli officials fear that these planes are loaded with advanced munitions, which could potentially be used against Israel.

The day before the strike, one such plane was spotted by a civilian skywatcher, using open-source flight tracking software, as it flew from Tehran toward an airfield in Hama.

Plane of Interest: Syrian Arab Air Force 585th Transport Squadron of the 29th Air Brigade’s four-engine turbofan strategic airlifter Ilyushin Il-76T YK-ATD flew from Tehran Mehrabad to -likely- Hamah military airport through Iraqi airspace. pic.twitter.com/AzqM3n5QlE — Yörük Işık (@YorukIsik) April 28, 2018

The blast from the air raid could be seen from kilometers away and registered a 2.6 on the Richter scale on nearby seismographs, likely from the munitions in the weapons depot and not the initial bombing.

An official from the coalition backing Syrian dictator Bashar Assad told the New York Times that the strike destroyed some 200 missiles. Former Military Intelligence chief Amos Yadlin said he believed they were surface-to-surface missile with heavy warheads — a type of weapon Israeli officials have said that Iran might try to bring into Syria.

Israel has refused to comment on the strike. Neither Syria nor Iran has publicly accused Israel of carrying it out, though many news outlets affiliated with them have pointed the finger at the Jewish state. Some have claimed that the United States or United Kingdom was behind the attack.

An Israeli satellite company revealed the damage caused to the site. Aerial photographs showed that at least 13 buildings were hit in the strike on the Hama base.

Speaking on condition of anonymity about the attack, the pro-Assad regime official said Tehran can be expected to hit back at Israel for the bombing, according to the New York Times.

However, Iran would likely wait to do so until after May 6 parliamentary elections in neighboring Lebanon, where its ally Hezbollah is fielding candidates, the official said.

Tehran has sent some 80,000 Iran-backed fighters to back Syrian President Bashar Assad’s forces in the country’s seven-year civil war, Israeli Ambassador to the United Nations Danny Danon said last week.

Israel has designated Iranian military entrenchment in Syria as unacceptable to it, fearing that Tehran could use the country as a springboard for attacks against the Jewish state.

Defense Minister Avigdor Liberman has repeatedly vowed that Israel will work to prevent this from occurring, “no matter the cost.”

The Sunday night attack comes amid soaring tensions between Iran and Israel following an airstrike earlier this month on Syria’s T4 air base in the central province of Homs that killed seven Iranian military personnel. Tehran has vowed to retaliate for the T4 attack.

Syria, Iran, and Russia blamed Israel for that T4 attack. Israel did not confirm or deny it.