A military media unit run by Hezbollah, a key Damascus ally, said the advancing forces reached an army garrison at the edge of the city.
The army and allied forces have made a swift lunge through militant lines in recent days to relieve an enclave in the Euphrates city, where Islamic State has besieged the garrison and some 93,000 civilians since 2014.
The Syrian army's breaking of a years-long siege by the Islamic State group of Deir al-Zor city is a "very important strategic victory," the Kremlin said on Tuesday.
"Commander-in-chief Vladimir Putin has congratulated the Russian military command (in Syria) as well as the command of the Syrian government troops with this very important strategic victory over the terrorists with the aim of freeing Syria from ISIL," Kremlin spokesman Dmitry Peskov told journalists.
Earlier Tuesday a Russian warship in the Mediterranean fired cruise missiles at IS fighters near the town of Al-Shula to aid the Syrian army, the Russian defence ministry said.
"As a result of these strikes there was damage to the infrastructure, underground communications, weapon stockpiles of the terrorists, and this allowed the armed contingents of government forces... to rapidly advance, break through IS defences and unblock the city (of Deir al-Zor)," Peskov said.
A Russian warship in the Mediterranean fired cruise missiles Tuesday toward IS group targets in the eastern Syrian province of Deir el-Zour. Source: AAP
Putin has also "sent a telegram to Syrian President Bashar al-Assad" praising the victory, he added.
Syria's army and allied fighters, backed by Russian air support, have been advancing towards Deir al-Zor on several fronts in recent weeks, and on Tuesday arrived inside the Brigade 137 base on its western edge.
Government forces and tens of thousands of civilians in the city have been trapped under the IS siege for over two years, facing food and medical shortages.
