The Syrian army and its allies have repelled a strong attack by ISIL fighters to capture a road linking Palmyra to Deir Az Zor city, a major supply route from government territory to the eastern city, according to a military media unit run by Lebanon's Hezbollah.

The ISIL assault, which began on Thursday, marked the first major counterattack against the allied forces since they broke through a swathe of territory to reach Deir Az Zor city earlier this month.

"The Syrian army and its allies completely secured the Deir az Zor-Palmyra highway after foiling the intense attack," the Hezbollah media unit said on Friday.

"The highway has become passable for traffic in both directions to and from Deir Az Zor," it added.

Hezbollah is one of the main allies of Syrian President Bashar al-Assad, alongside Russia, Iran and other pro-government militias.

Earlier on Friday, a commander in the military alliance backing Assad said the road linking Deir Az Zor to Palmyra was only being used in cases of absolute necessity and that the army and its allies were fighting to recover lost ground.

Helped by the Russian military and Iran-backed militias, the Syrian army's advance this month to Deir Az Zor lifted a three-year-long siege imposed by ISIL on a government-held enclave in the city.

Deadly ISIL attacks

The Syrian Observatory for Human Rights, a UK-based monitor tracking developments in Syria's conflict, said the ISIL attacks had killed more than 58 fighters from the Syrian army and allied forces since Thursday.

The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group said on Thursday it had killed around 100 government fighters south of the town of al-Sukhna, which is also located on the road to Deir Az Zor, and announced it had taken a hill overlooking the town.

A US-backed alliance of Syrian militias, the Syrian Democratic Forces, is waging a separate offensive against ISIL in Deir Az Zor to the east of the Euphrates River, which bisects the province.

ISIL leader Abu Bakr al-Baghdadi purportedly pressed his followers to "stand fast" and keep fighting, in an undated recorded statement released on Thursday.

The date of the 46-minute recording, released on Thursday via the ISIL-linked Al-Furqan news organisation, was not clear.

But in it, the ISIL leader referred to the North Korean threats against Japan and the US.

READ MORE: ISIL posts 'Baghdadi audio' issuing 'resistance' call

It is the first apparent communication from the elusive leader in almost year, during which the group has lost much of the territory it controlled in Iraq and Syria.

The audio release, much of which is dedicated to religious scriptures, comes amid growing speculation over al-Baghdadi's fate.

"The leaders of [ISIL] and its soldiers have realised that the path to ... victory is to be patient and resist the infidels," he said in the message.

In a statement to AFP news agency, a US intelligence official suggested the tape could well be real but added that it was still being verified.

'Worst string of attacks'

On Wednesday, the New York-based Physicians for Human Rights (PHR) accused Russia and the Syrian government of mounting the "worst string of attacks" on hospitals in Syria since April despite an agreement on "de-escalation" zones.

The group said it believes either Russian or Syrian government jets were behind at least five air raids on three hospitals in Syria's rebel-held Idlib province.

Three facilities came under fire on the same day, September 19, and two of the hospitals were bombarded again separately days later, PHR reported.

Meanwhile, the UN Security Council met to discuss the humanitarian crisis in Syria on Wednesday.

READ MORE: Syria's civil war explained from the beginning

British and French diplomats described the recent escalation as "unacceptable and appalling".

UN special envoy Staffan de Mistura said he hopes to convene an eighth round of Syria talks in Geneva no later than early November.

Syrian ambassador to the UN, Bashar al-Jaafari, the government's chief negotiator, was also present at the Security Council chamber.

"I am calling on both sides to assess the situation with realism and responsibility to the people of Syria and to prepare seriously to participate in the Geneva talks," de Mistura said in New York.