Russian submarines have fired seven cruise missiles at Isis militants in Syria, Russia’s defence ministry reportedly said.

The craft were operating in the eastern Mediterranean, news agencies said, and used Kalibr missiles to strike targets in the suburbs of Deir Ezzor, in Syria’s east.

“The targets were command posts, communication centres, as well as militants’ weapons and ammunition stockpiles in areas of south-east Deir Ezzor under the control of Islamic State,” the ministry said.

The submarines were the Kilo-class boats Velikiy Novgorod and Kolpino, according to Russian state media.

The head of the Britain-based Syrian Observatory for Human Rights said buses and vehicles carrying about 400 militants and civilians crossed into Deir Ezzor province on Wednesday.

Syrian government forces broke a three-year Isis siege on parts of the provincial capital last week, and are now battling the extremists inside the city.

The militants control less than half the city and are encircled on three sides with their backs to the Euphrates River. However, they still control rural areas outside the city and the border with Iraq.

US-backed Syrian forces are meanwhile advancing in the surrounding province from the east and north, on the other side of the river, setting up a race to the border with Iraq.

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Bassem Aziz, a spokesman for the US-backed and Kurdish-led Syrian Democratic Forces, said they have taken control of an industrial area on the eastern bank of the river, a few miles from the government troops.

Mr Aziz said they were about four miles away from the city's eastern entrance.