Russia's military fired seven cruise missiles Thursday at ISIS targets in the eastern Syrian province of Deir el-Zour as pro-government forces closed in on the militants holed up in the eponymous capital.

Journalists on a trip organized by the Russian Defence Ministry watched from the deck of Russia's Admiral Essen frigate as two submarines launched seven missiles from the Mediterranean Sea.

The Syrian government forces, backed by Russian air cover, last week broke a three-year siege around the city on the Euphrates river.

Ministry spokesman Maj. Gen. Igor Konashenkov told reporters later that intelligence showed the missiles hit the targets southeast of Deir el-Zour, destroying an Islamic State in Iraq and Syria (ISIS) command centre, a communications hub, an ammunition depot and an unspecified number of fighters.

Russia has provided military backing for Syrian President Bashar al-Assad's forces since 2015. It has repeatedly fired salvoes of such missiles, from both the sea and mainland Russia.

An Associated Press reporter on the deck of the Admiral Essen frigate saw three missiles and later four more flying into the air, leaving trails of smoke. Two submarines have emerged and were visible on the horizon shortly afterwards.

Chris Brown of CBC News was among the Western journalists on the ship to witness the launch.

"The fact that Russia's military brought so many foreign journalists so far into the ocean to see this is meant to be a demonstration of their confidence in what they've been doing in Syria from a military point of view," said Brown.

Major gains recently

Backed by an intense aerial campaign, Syrian and allied forces pushed their way toward the city last week, breaking a nearly three-year siege on its troops on the western edge of Deir el-Zour. It was a major symbolic victory for the pro-government forces.

Since then, they have been battling remnants of the militants inside the city, seizing more than 60 per cent of it. On Thursday, the pro-government forces were closing in at the extremists from three sides along the river, pounding al-Bogheliyah neighbourhood on the northwestern edge of the city.

Syria Russia

In this image made from video a long-range Kalibr cruise missile is launched by the Russian Navy submarine in the Mediterranean on Thursday. Russia's military says it fired seven cruise missiles at Islamic State targets in the eastern Syrian province of Deir el-Zour. (Associated Press)

The militants are currently encircled by Syrian troops from three sides, with their backs to the Euphrates River. However, they still control rural areas outside the city and the border with Iraq.

As ISIS reels from significant losses in Syria and Iraq, there is a race for control of the border with Iraq, currently still in the militants' hands. U.S.-backed Syrian forces are meanwhile advancing in the surrounding province from the east and north, on the other side of the river.

Bassem Aziz, a spokesman for the U.S.-backed and Kurdish-led Syrian Democratic Forces, said his troops have taken control of an industrial area on the eastern bank of the river, a few miles from the government troops. Aziz said they are about six kilometres away from the city's eastern entrance.

Syria-Lebanon deal

In its statement last week, the U.S-led coalition said it will back its partners on the ground to defeat ISIS and "will do our utmost to ensure that [ISIS] terrorists do not move toward the border of our Iraqi partners."

Overnight, a convoy of Islamic State militants and their relatives being evacuated from the border with Lebanon has crossed into Deir el-Zour from a desert area in central Syria, ending a standoff with the U.S-led coalition that briefly overshadowed the race for the province.

The evacuation, negotiated by Lebanon's Hezbollah group, removed the militants from the Syria-Lebanon border but angered Iraq and the U.S., which said they should have been killed on the battlefield not moved to the Iraq border.

The deal reached at the end of August allowed hundreds of militants and their families to relocate to Boukamal, an ISIS-held Syrian town near the Iraqi border, in exchange for ISIS-held prisoners and the remains of Lebanese soldiers captured in 2014. One surviving Hezbollah fighter was returned to Lebanon Thursday.

The head of the Britain-based Syrian Observatory for Human Rights Rami Abdurrahman said buses and vehicles carrying about 400 militants and civilians crossed into Deir el-Zour province Wednesday. It was not clear where the buses went.

The U.S.-led coalition struck the road the convoy was traveling on, leaving it stranded in the desert for about two weeks, though some vehicles were able to slip into militant-held territory. The U.S. said it did not strike the convoy itself because of the presence of civilians.

Last week, the U.S.-led coalition said it ended surveillance of the convoy after a Russian request, as Syrian troops advanced against ISIS in eastern Deir el-Zour province.

The proximity of the two forces raises the spectre of confrontation, as both sides vie for the border with Iraq and the oil and resources-rich province.