The United States has described as �unfortunate but not surprising� Russia�s announcement of using an Iranian airbase to launch airstrikes against terrorists in Syria.

US State Department Spokesman Mark Toner made the comments on Tuesday, saying, Washington is still assessing the extent of the Russian-Iranian cooperation over Syria�s conflict.

Washington�s reaction came after the Russian state media earlier said that the airstrikes had been carried out from an airbase in northwestern Iran, as part of a cooperation agreement between Moscow and Tehran in an effort to battle the Daesh Takfiri terrorist group operating in Syria.

Toner said Moscow�s use of an Iranian airbase would not necessarily prevent the US from reaching a deal with Russia to cooperate in the fight against the Takfiri group.

The two sides have yet to reach an agreement, he noted.

The State Department spokesman further stressed Washington�s concern over Russia�s attacks on what the US regards as moderate militants in Syria.

Since March 2011, Syria has been gripped by militancy it blames on some Western states and their regional allies.

The Takfiri militants operating in the Middle Eastern state have suffered major setbacks over the past few months as the Syrian army has managed to liberate several areas.

Russia has been assisting Syria in its fight�against militants since late September 2015 upon a request by the Damascus government. Moscow says the sorties have inflicted huge losses on militants in east and northeast of Syria.

Last month, Syrian President Bashar al-Assad hailed the Russian air campaign in support of the Syrian army, saying it has helped Syrian soldiers advance against the Takfiri elements operating in the Arab country.

According to an estimate by United Nations Special Envoy for Syria Staffan de Mistura, over 400,000 people have been killed in the Syrian war�since it broke out in March 2011.

By Press TV