Russia’s President Vladimir Putin has congratulated his counterpart Bashar al-Assad after Syrian state media said government troops broke a three-year long siege of the eastern city of Deir al-Zour by Islamic State forces.

Putin hailed the announcement on September 5 as an "important strategic victory" in the conflict, Kremlin spokesman Dmitry Peskov told reporters in Moscow.

Syrian government forces and tens of thousands of civilians in the city have been trapped by the IS siege for more than two years, suffering under food and medical shortages.

Syria's official news agency Sana reported that the army and allied militiamen had joined up with forces at the Brigade 137 base on Deir al-Zour's outskirts.

IS militants were driven out of their stronghold in the Iraqi city of Mosul earlier this year.

Islamic State still holds half of Deir al-Zour city and large swaths of the province, however, as well as parts of its former stronghold Raqqa to the northwest, where the U.S.-backed offensive is being fought.

Earlier on September 5, a Russian warship in the Mediterranean Sea fired cruise missiles at IS stations near the town of Al-Shula to aid the Syrian army, the Russian Defense Ministry said.

"As a result of these strikes, there was damage to infrastructure, underground communications, weapon stockpiles of the terrorists, and this allowed the armed contingents of government forces... to rapidly advance, break through IS defenses and unblock (Deir al-Zour)," Peskov said.

Based on reporting by AP, AFP, Reuters, and Interfax