It was signed between Chief of Staff of the Iranian Armed Forces Mohammad Baqeri and Syrian Defense Minister Ali Abdullah Ayyoub in Damascus, reports Xinhua quoting a semi-official Fars news agency.
As part of the agreement, Iran "will strengthen Syria's air defense system," Baqeri said, adding that "the agreement will increase our determination for joint cooperation to confront the US pressures".
The people do not welcome the US presence in the region, he noted.
The Iranian commander also urged Turkey to settle problems with Syria through dialogue.
For his part, Ayyoub said that the US has been "unable to bring Iran, Syria and the resistance front to their knees."
Iran has been a major ally of the Syrian government in its fight against the armed rebels since 2011.