The leader of the Islamic State organisation won the allegiance of Egypt's deadliest militant group Monday as Iraqi authorities investigated reports he had been killed or wounded in a US air strike.

Syrian President Bashar al-Assad meanwhile said he was ready to study a UN plan for local ceasefires in the battle-scarred northern city of Aleppo.

Egypt's Ansar Beit al-Maqdis, which has carried out a string of deadly attacks from its Sinai stronghold, pledged loyalty to Abu Bakr al-Baghdadi -- the self-declared IS "caliph".

Speculation has swirled that Baghdadi, whose group has seized large parts of Iraq and Syria, was killed or wounded in a US strike Friday on jihadist leaders in northern Iraq, but there has been no confirmation.

In a recording posted on Twitter, Ansar Beit al-Maqdis promised its loyalty to IS and urged other Muslims to do the same.

"We announce our pledge of allegiance to the caliph Ibrahim Ibn Awad... to listen and obey," the audio recording said, using another name for Baghdadi.

"We call on all Muslims everywhere to pledge allegiance to the caliph and support him."

It was the most significant vow of support for IS in the region outside Iraq and Syria, suggesting its influence over militant groups is overshadowing its once dominant Al-Qaeda rivals.

- Strike hit IS gathering -

Ansar Beit al-Maqdis, which means "Partisans of Jerusalem", has killed scores of police and soldiers since Egypt's army ousted Islamist president Mohamed Morsi last year.

IS declared a "caliphate" in areas under its control in June, imposing its harsh interpretation of Islamic law and committing widespread atrocities.

Concerned after it overran large parts of Iraq from its previous base in Syria, Washington forged a coalition of Western and Arab allies to launch air raids against the group in both countries.

The US military said in its latest update that the coalition had carried out 23 strikes in Syria and 18 in Iraq between Friday and Monday.

The Pentagon said strikes late Friday hit a gathering of IS leaders in areas of the northern Iraqi city of Mosul, sparking reports Baghdadi may have been killed or wounded there, or in western Anbar province.

The death of the elusive IS leader would be a major victory for the coalition, but US and Iraqi officials have so far been unable to confirm he was targeted.

It is unclear whether officials even know if he was at the gathering, given the dearth of intelligence from IS-held areas of Iraq and Syria.

The Pentagon said Monday the US "cannot confirm" Baghdadi's status, suggesting last week's strike had hit lower "tactical level leadership" of IS.

It also said 50 US troops have deployed to Iraq's Anbar province for the first time in the fight against IS, preparing the way for a larger contingent.

US President Barack Obama, who has unveiled plans to send up the 1,500 Americans to Iraq to advise and train its forces, said Sunday the battle against IS was entering a "new phase" after months of air strikes.

Obama, who pulled US forces from Iraq in 2011, again vowed they would not be engaged in combat under the new mission.

- Assad 'to study' ceasefire -

The battle against IS has in recent months overshadowed the civil war in Syria, where disparate rebel groups are fighting to overthrow President Bashar al-Assad.

Assad said Monday he was ready to consider a plan from UN envoy Staffan de Mistura to "freeze" fighting in Aleppo city to allow for aid deliveries and lay the groundwork for broader peace talks.

"President Assad has been informed by de Mistura of the main points of his initiative," his office said.

"(Assad) said it was worthy of study and that work on it is needed... in order to re-establish security in Aleppo."

But Washington cast doubt on the report, with State Department spokeswoman Jen Psaki saying similar "truces thus far have more closely resembled surrender arrangements, as opposed to genuine, sustainable ceasefire arrangements".

Aleppo, once Syria's economic capital, has been split into rebel- and army-held areas since a major insurgent offensive began there, with near-daily air raids targeting rebel-held districts and reportedly killing mostly civilians.

A report overseen by the Britain-based Madani civil society group said local ceasefires would be the best way forward in Syria.

The report said such truces "offer the best hope of alleviating the suffering of the Syrian people -- by reducing levels of violence, providing safe havens within Syria and offering access to humanitarian assistance".

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