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Iran on Monday accused Israel of "flagrant" aggression in Syria after an air strike hit a regime base in Homs, killing four Iranian "military advisers", AFP reported.

The Islamic Republic condemned "strongly the aggression and the air strike by the Zionist regime" against the T-4 airbase in Homs, Iran's foreign ministry said in a statement.

The ministry called on "all free states... to not be indifferent to this flagrant Zionist aggression", adding that the air strike further complicated the situation in the war-torn country.

Iran's Fars news agency said earlier that "four Iranian military advisers" were killed in attack on the military airport early Monday morning.

The news agency, which is close to the country's elite Revolutionary Guards, had earlier reported that three Iranians had "fallen as martyrs".

The ministry's press release made no mention of the alleged victims.

The Syrian Observatory for Human Rights, which monitors the country's conflict, said 14 fighters were killed, including Syrian army officers and Iranian forces.

Iran has long been a strong supporter of Syrian President Bashar Al-Assad and has been providing him with both financial aid and military advisors against a range of opposing forces.

Near the start of the Syrian civil war, it was reported that then-Iranian President Mahmoud Ahmadinejad had personally sanctioned the dispatch of officers from the Iranian Revolutionary Guards to Syria to fight alongside Assad’s troops.

A senior Iranian commander made clear last year that Iran will provide military advisors to Syria for as long as necessary and stressed that “the advisory help isn't only in the field of planning but also on techniques and tactics.”

The Syrian regime and Moscow also blamed Israel for carrying out Monday's strike.

The IDF has refused to comment on the air strike. Prime Minister Binyamin Netanyahu on Monday said that Israel reserved the right to use preemptive strikes against threats to its security, though he did not directly say Israel was behind the air strike in Homs.

“We have one clear and simple rule and we seek to express it constantly: If someone tries to attack you – rise up and attack him," said Netanyahu, referencing the Talmudic dictum of “when someone plans to kill you, get up early and kill him first”.