Iran blasted US concern over deepening ties between Tehran and Damascus with a scathing outburst against the "evil regime".

“The anger of an evil regime, whose wind was taken out in Syria and the entire region by Iran and the axis of resistance is natural,” Iran’s Foreign Ministry Spokesman Nasser Kanaani blasted on Thursday.

Kanaani threatened that Washington must end its “aggressive presence” in war-torn Syria.

The US State Department warned Wednesday that closer ties between Iran and Syria should be of great concern not only to US allies, but also to the world more broadly.

"These are two regimes that have continued to partake in malign destabilizing activities, not just in their immediate countries, but also in the region," State Department Spokesperson Vedant Patel said.

That same day, Iranian President Ebrahim Raisi met his Syrian counterpart Bashar al-Assad in Damascus in his first visit since Syria's war began in 2011, underlining close ties as Syrian relations with Arab states thaw.

In an interview with pro-Iran broadcaster al-Mayadeen before his trip, Raisi said he planned to "consolidate and develop" ties with Syria and other allies, including the Lebanese armed group Hezbollah.

Assad and Raisi signed a memorandum of understanding on long-term comprehensive strategic cooperation while the delegates signed 15 separate documents.

Raisi's visit comes as Iran and regional rival Saudi Arabia rebuild relations after years of tensions, and as Arab states that shunned Assad - including Riyadh - rebuild ties with his government.