Russia's Foreign Ministry has said there is a high degree of certainty that Isis leader Abu Bakr al-Baghdadi is dead.

A new statement carried by RIA news agency on Thursday doubled down on Moscow's claim last week that its forces may have killed the secretive Islamist leader.

Baghdadi is believed to be in hiding in the region around the Syria-Iraq border, moving frequently and avoiding telecommunications to evade detection. According to the Russian foreign ministry, the strike in which he was allegedly killed occurred at the end of May.

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The Russian authorities have not produced any proof since the initial claim was made. Washington DC said it could not corroborate the death and Western and Iraqi officials have been sceptical.

"His death has been reported so often that you have to be cautious till a formal [Isis] statement comes," a European security official speaking on condition of anonymity told Reuters.

On Tuesday, a spokesperson for the Russian Foreign Ministry appeared to backtrack on the initial claim, saying Baghdadi's death "could not be verified".

The Trump administration has also thrown cold water on the claims that two other senior Isis leaders and up to 300 foot soldiers were killed in Russian strikes around the same time as the alleged death of Baghdadi, noting "infirmities" in the reports.

“A strike of that size and that claim that would have happened that long ago without any knowledge is something that made me curious,” an official said.

In pictures: Isis' weapons factories Show all 11 1 /11 In pictures: Isis' weapons factories In pictures: Isis' weapons factories A mortar round fin manufactured by Isis in Gogjali, Mosul, November 2016 Conflict Armament Research In pictures: Isis' weapons factories Isis rocket components discovered in Gogjali, Mosul, Iraq in November 2016 Conflict Armament Research In pictures: Isis' weapons factories Isis mortars discovered near Karamlais, Iraq, in November 2016 CAR In pictures: Isis' weapons factories An Isis rocket launch frame in Qaraqosh, November 2016 Conflict Armament Research In pictures: Isis' weapons factories A memo from Isis' COSQC on quality control at a manufacturing facility in Gogjali, Mosul, November 2016 Conflict Armament Research In pictures: Isis' weapons factories Electrically-operated initiators manufactured by Isis in forces Gogjali, Mosul, November 2016 Conflict Armament Research In pictures: Isis' weapons factories Isis mortar tubes at a manufacturing facility in Karamlais, November 2016 Conflict Armament Research In pictures: Isis' weapons factories An Isis mortar production facility discovered in Gogjali, Mosul, in November 2016 Conflict Armament Research In pictures: Isis' weapons factories An Isis weapons manufacturing facilities near Mosul in November 2016 Conflict Armament Research In pictures: Isis' weapons factories Stocks of French-manufactured Sorbitol, Latvian potassium nitrate and Lebanese sugar at an Isis weapons factory in Iraq Conflict Armament Research In pictures: Isis' weapons factories A destroyed Isis weapons facility in Qaraqosh, Iraq, November 2016 Conflict Armament Research

However, if the report does prove to be true, it would be a huge blow to the extremist organisation, which is rapidly losing territory across both Syria and Iraq.

The US has also claimed that both Isis' grand mufti, or preacher, Turki Binali, and the head of its propaganda wing, Amaq Rayaan Mashal, have been killed in recent strikes.

Baghdadi gained international prominence when he declared the creation of Isis' so-called "caliphate" from Mosul's grand mosque after his forces swept across northern Iraq from Syria in the summer of 2014. He has not made a public appearance since.

On Wednesday night the 12th century al-Nuri mosque was blown up by militants wishing to deprive US-backed Iraqi coalition forces of claiming victory from the same spot the caliphate was created.

There are believed to be fewer than 300 militants left in Mosul's Old City, where the battle to defeat Isis is reaching its endgame after nine months of bloody fighting.

Across the border in Syria, the struggle to oust Isis from its de facto capital of Raqqa is in its early stages after US-backed Kurdish troops took back four neighbourhoods earlier this month.