At least nine Russians and 26 Syrian soldiers have died after a ferocious attack by Isis militants in Syria’s eastern desert, reports say.

The Russian ministry of defence said at least four Russian national soldiers were killed by rebel shelling during a joint attack mounted by several different opposition groups on an army base in Mayadeen in Deir Ezzor province last Wednesday. 

Two Russian military advisers were killed during the incident and two more later died in hospital, Russian news outlets reported. 

UK-based war monitor the Syrian Observatory for Human Rights put the total of fighters killed at 35. 

“There were 35 pro-government forces killed, including at least nine Russians. Some of those Russian nationals were government troops, but not all of them,” SOHR head Rami Abdel Rahman said. 

According to the Russian defence ministry, 43 attackers were killed during last week’s fighting. 

The Syrian army recaptured Deir Ezzor city from Isis in November 2017, removing the militants from their last urban stronghold in the country. 

The group have since switched to guerrilla-style fighting tactics to combat both pro-Syrian government forces and US-backed Kurdish fighters in the region. 

The attack comes after a recent spate of Isis posters and videos warning Russian President Vladimir Putin he will “pay the price for killing Muslims” ahead of the 2018 Fifa World Cup in Moscow, which begins next month.

Several images have emerged on channels used by extremists on secure messenger service Telegram of the Russian leader either as the subject of target crosshairs or tied up on a chair. 

The official number of Russians killed since the country began military intervention in the complex civil war on behalf of embattled President Bashar al Assad in 2015 now stands at 92. 

Russian air and ground support has been instrumental in turning the tide of the now seven-year-old war in President Assad’s favour.

It is believed hundreds of privately contracted Russian mercenaries are also stationed in Syria. At least 200 are thought to have been killed in fighting to date but are not counted among the official toll. Many of that number are believed to have died in US-led bombings in Deir Ezzor. 

On Monday, Russia’s foreign minister, Sergey Lavrov, told reporters in Moscow that “only Syrian forces” should be present in Syria’s southern border areas with Israel and Jordan.

The comments have widely been interpreted as revealing increasing Russian impatience with the rising Israeli-Iranian tensions in Syria.

Israel’s defence minister, Avigdor Lieberman, is travelling to Moscow for meetings later this week. 

Israel has reiterated multiple times it will not tolerate a permanent Iranian military presence in the devastated country.