Damascus and Tehran must work against the influence of the Zionist regime of Israel and the United States, he asserts.
The Syrian Prime Minister Wael al-Halqi submitted the letter to the Iranian side on Tuesday as he was on a trip to Tehran.
Assad asserted in the letter that the 2 countries share a common will to develop mutual ties on all levels in the face of the Zionist-American schemes.
Also during his meeting with president Rouhani, Halqi praised Iran for its support for Syria during the recent crisis in the Arab country.
Iran, which a close ally to Syria, has been supporting Syria’s legitimate government in countering the foreign-backed terrorist militants who have been waging war and terror acts in the country for over 3 years.
Syria has been afflicted by civil war since March 2011 with Islamic State in Iraq and the Levant (ISIL) terrorists currently controlling mostly eastern parts of the country.
The US along with its regional allies have been supporting the terrorist militants operating inside Syria since then.