"We will strengthen Syria's air defence systems in order to improve military cooperation between the two countries," the broadcaster quoted armed forces Chief of Staff Major General Mohammad Bagheri as saying.

The deal will "further enhance our will... to confront pressures by America," he added.

Key US ally Israel, Iran's arch-enemy, has launched hundreds of strikes in Syria since the start of a civil war there in 2011, targeting government troops, allied Iranian forces and fighters from the Iran-backed Lebanese Shiite militant group Hezbollah.

Iran has always denied sending forces to fight in Syria, saying it only has military advisors there.

The broadcaster published on its website a video of the deal being signed in Damascus by Bagheri and Syria's Defence Minister Ali Ayoub.

It shows an Iranian military spokesman reading a joint statement saying the "comprehensive military-security agreement" aims to "confront increasing dangers (and) takfiri terrorism supported by international and regional powers".

Iranian authorities use the term "takfiri" to refer to Sunni jihadists, including groups fighting in Syria.

The joint statement Wednesday also emphasised "the necessity of the withdrawal of all foreign armed forces having entered Syria illegally".

Along with Moscow, Tehran is one of Syrian President Bashar al-Assad's main allies in the war that has ravaged the country since 2011.

The conflict has killed more than 380,000 people and displaced millions.