The US military has said it may have unintentionally struck Syrian troops while carrying out a raid against Isis on Saturday, which if confirmed would mark the first known direct American strike on President Bashar al-Assad's forces.

US Central Command said the strike was immediately halted “when coalition officials were informed by Russian officials that it was possible the personnel and vehicles targeted were part of the Syrian military.”

The Syrian military said the air strike hit a base in the eastern city of Deir Ezzor that is surrounded by Isis, allowing the extremists to advance. Russia's military said it was told by the Syrian army that at least 62 soldiers were killed in the air raid and more than 100 wounded.

The apparently errant strike could deal a crushing blow to a fragile US and Russian-brokered cease-fire that has largely held for five days despite dozens of alleged violations on both sides. The cease-fire, which does not apply to attacks on Isis, has already been the subject of disputes between Moscow and Washington, with each accusing the other of failing to fully implement it.

“Coalition forces would not intentionally strike a known Syrian military unit,” the military statement said.

The Syrian military said the air strikes enabled an Isis advance on a hill overlooking the air base. It called the strike a “serious and blatant attack on Syria and its military,” and “firm proof of the US support of Daesh [Isis] and other terrorist groups”. President Assad's government views all those fighting against it as “terrorists,” and has long accused the U.S. and other rebel supporters of backing extremists.

A Russian Defense Ministry official said Syria has informed them that 62 of its soldiers were killed in the air strike. Russia has been waging a year-old air campaign on behalf of Assad's forces and closely coordinates with them.

Syria war: Heavy fighting ahead of ceasefire

Defence Ministry spokesman Maj. Gen. Igor Konashenkov said the airstrike near Deir Ezzor airport was conducted by two F-16s and two A-10s. He did not identify the planes' country affiliation, but said they were part of the international coalition. The planes came from the direction of the Iraqi border, he added.

He said Isis fighters surrounding the air base launched an attack on the Syrian army positions after the air strike. He added that if the coalition attack was launched by mistake, the reason for it was a “stubborn reluctance by the American side to coordinate its action against terrorist groups in Syria with Russia.”

Russia has called for an emergency session of the United Nations Security Council over over the US-led raid. Spokeswoman Maria Zakharova says Moscow is demanding “full and detailed explanations about whether this was deliberate support of the Isis or another mistake.”

Ms Zakharova was quoted by the state news agency Tass as saying that “after today's attack on the Syrian army, we come to the terrible conclusion that the White House is defending the Islamic State [Isis].”

In pictures: Russian air strikes in Syria Show all 19 1 /19 In pictures: Russian air strikes in Syria In pictures: Russian air strikes in Syria Syrian boys cry following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo Getty In pictures: Russian air strikes in Syria Russian defense ministry spokesman Major General Igor Konashenkov speaks to the media in Moscow, Russia. Konashenkov strongly warned the United States against striking Syrian government forces and issued a thinly-veiled threat to use Russian air defense assets to protect them AP In pictures: Russian air strikes in Syria Syrians wait to receive treatment at a hospital following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Alepp Getty In pictures: Russian air strikes in Syria Russian Deputy Defense Minister Anatoly Antonov speaks at a briefing in the Defense Ministry in Moscow, Russia. Antonov said the Russian air strikes in Syria have killed about 35,000 militants, including about 2,700 residents of Russia AP In pictures: Russian air strikes in Syria Jameel Mustafa Habboush, receives oxygen from civil defence volunteers, known as the white helmets, as they rescue him from under the rubble of a building following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo Getty In pictures: Russian air strikes in Syria Civil defence members rest amidst rubble in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria Reuters In pictures: Russian air strikes in Syria A girl carrying a baby inspects damage in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria Reuters In pictures: Russian air strikes in Syria Civilians and civil defence members look for survivors at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria Reuters In pictures: Russian air strikes in Syria Civilians and civil defence members carry an injured woman on a stretcher at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria Reuters In pictures: Russian air strikes in Syria Volunteers from Syria Civil Defence, also known as the White Helmets, help civilians after Russia carried out its first airstrikes in Syria In pictures: Russian air strikes in Syria The aftermath of Russian airstrike in Talbiseh, Syria In pictures: Russian air strikes in Syria Smoke billows from buildings in Talbiseh, in Homs province, western Syria, after airstrikes by Russian warplanes AP In pictures: Russian air strikes in Syria Russian Air Forces carry out an air strike in the ISIS controlled Al-Raqqah Governorate. Russia's KAB-500s bombs completely destroy the Liwa al-Haqq command unit In pictures: Russian air strikes in Syria Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria Â© TASS/ITAR-TASS Photo/Corbis In pictures: Russian air strikes in Syria Russia claimed it hit eight Isis targets, including a "terrorist HQ and co-ordination centre" that was completely destroyed In pictures: Russian air strikes in Syria A video grab taken from the footage made available on the Russian Defence Ministry's official website, purporting to show an airstrike in Syria In pictures: Russian air strikes in Syria A release from the Russian defence ministry purportedly showing targets in Syria being hit In pictures: Russian air strikes in Syria Russia launched air strikes in war-torn Syria, its first military engagement outside the former Soviet Union since the occupation of Afghanistan in 1979. Russian warplanes carried out strikes in three Syrian provinces along with regime aircraft as Putin seeks to steal US President Barack Obama's thunder by pushing a rival plan to defeat Isis militants in Syria In pictures: Russian air strikes in Syria Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria, a thousand kilometres away. The targets include ammunition factories, ammunition and fuel depots, command centres, and training camps Â© TASS/ITAR-TASS Photo/Corbis

Isis has repeatedly attacked the government-held air base, which is an isolated enclave deep in extremist-held territory.

The US-led coalition has carried out thousands of airstrikes against Isis in Syria and Iraq over the past two years, allowing allied forces on the ground to liberate several towns and cities from the extremist group. Russia also carries out attacks against IS targets, in Deir el-Zour and other parts of Syria.

“The coalition will review this strike and the circumstances surrounding it to see if any lessons can be learned,” the U.S Central Command statement said.

The cease-fire took effect on Monday, and despite reports of violations, it has largely held. However, aid convoys have been unable to enter rebel-held parts of the northern city of Aleppo - a key component of the deal.

UN makes Syria aid passage plea

Earlier on Saturday, Russian President Vladimir Putin questioned the US commitment to cease-fire, suggesting that Washington wasn't prepared to break with “terrorist elements” battling Assad's forces.

Russia has accused Washington of failing to rein in the rebels, and on Saturday Mr Putin asked why the United States has insisted on not releasing a written copy of the agreement.

“This comes from the problems the US is facing on the Syrian track — they still cannot separate the so-called healthy part of the opposition from the half-criminal and terrorist elements,” Mr Putin said during a trip to Kyrgyzstan.

“In my opinion, this comes from the desire to keep the combat potential in fighting the legitimate government of Bashar Assad. But this is a very dangerous route.”

He appeared to be referring to Jabhat al-Nusra a group who recently claimed to have ended their links with al-Qaeda-linked and changed their name to Jabhat Fatah al-Sham. The group is deeply embedded in rebel-held areas and fights alongside more moderate groups. Abu Mohammed al-Golani, the leader of the group, condemned the ceasefire agreement in an interview with Al-Jazeera TV aired on Saturday.

Russian Foreign Minister Sergey Lavrov echoed Mr Putin's remarks during a phone call with US Secretary of State John Kerry. Mr Lavrov noted the “refusal by an array of illegal armed groups to join the cease-fire,” and Washington's obligation to “separate units of the moderate opposition from terrorist groupings,” according to a Foreign Ministry statement.

Under the ceasefire agreement, the US and Russia would work together to target al-Nusra, as well as Isis, while Assad's forces refrain from striking opposition-held areas.

But Washington has warned Russia that unless aid is delivered to Aleppo, it will not move ahead with the formation of the joint coordination center.

The UN has accused Assad's government of obstructing aid access to the contested city. The Russian military says insurgents have held up the delivery by firing on government positions along the main route leading into besieged, rebel-held districts, in violation of the cease-fire.

The Syrian government said it has done all that is necessary to facilitate the entry of aid convoys to Aleppo, but that armed groups have failed to withdraw from the supply routes and are committing “dangerous, provocative acts.”

Russia's military said on Saturday that Syrian rebels violated the ceasefire dozens of times over the past day, including with strikes on military and civilian targets in Aleppo. Syrian activists said government forces killed five civilians in separate attacks.

Syria's conflict has killed more than 300,000 people and displaced half the country's population since March 2011.