Thirteen members of Iran's elite Revolutionary Guard were killed in a battle with Islamist militants over a village near the Syrian city of Aleppo, Tehran said on Saturday, in one of Iran's biggest single-day losses since it sent forces to support President Bashar al-Assad.

Russia meanwhile said that a truce in Aleppo itself had been extended until Monday.

Islamist forces seized the village of Khan Touman, about 15 kilometres southwest of Aleppo, on Friday and dozens of people were reported to have been killed in the fighting.

The attack was launched by an alliance of Islamist insurgents known as Jaish al-Fatah, including the al-Qaeda-linked Nusra Front.

Iran's Fars news agency quoted a Revolutionary Guard official as saying that 13 Iranian military advisers had been killed and 21 others wounded.

The British-based Syrian Observatory for Human Rights said it had confirmed from its sources on the ground the death of 20 Iranians, including 13 advisers.

The monitoring group said that among the Iranian-backed militia fighters involved in the battles, six from Lebanon's Hezbollah Shia movement and 15 Afghan Shia fighters were also killed.

Jaish al-Fatah and affiliates posted videos and photos on social media of what appeared to be the bodies of Iranians or Shia militiamen who were killed in Khan Touman. They included footage of wallets, personal documents and Iranian currency.

Members of Iran's elite Revolutionary Guard march during a parade in this 2008 file photo. (Vahid Salemi/Associated Press)

Iran, along with Russia, has been a principal ally of Assad in the five-year-old civil war, while Gulf Arab states and the West have supported various rebel factions.

A senior adviser to Iran's Supreme Leader on Saturday reiterated Tehran's continued support of Assad in a meeting with the president in Damascus.

"Iran will use all its means to fight against terrorists who are committing crimes in the region," Ali Akbar Velayati, Ayatollah Khamenei's adviser on international affairs, was quoted as saying by Fars.

Iranian proxies, including Afghans and Iraqis ,as well as Lebanese, have been involved in Syria from as early as 2012.

While Tehran previously said its support was limited to advisers, it has been more open about the extent of its role since Russia intervened on Assad's side last year.

Iran has been particularly involved in campaigns around Aleppo in northwest Syria, which was the country's commercial and industrial centre before the war and is now divided between government and rebel forces.

Fighting in the countryside to the south of Aleppo has escalated in recent days despite a ceasefire in city itself.

The Russian defence ministry said a "regime of calm" truce in Aleppo and parts of Latakia province had been extended for 72 hours, beginning at 1 a.m. on Saturday, Syrian state news agency SANA reported.

A Red Crescent aid worker inspects scattered medical supplies after an airstrike on a medical depot in a rebel-held neighbourhood of Aleppo, Syria on April 30, 2016. (Abdalrhman Ismail/Reuters)

The Syrian Observatory for Human Rights said Syrian and Russian warplanes had intensified their bombing of insurgent positions near Khan Touman.

In northern Syria, U.S.-led coalition airstrikes killed 48 Islamic State fighters on Saturday, Turkey's state-run Anadolu Agency said, quoting the Turkish military.

The strikes were in response to increasingly frequent Islamic State attacks against opposition forces in the area, Turkisk security sources told Andadolu.

The Turkish border town of Kilis, which lies just across the frontier from Islamic State-controlled territory of Syria, has been hit by regular rocket fire in recent weeks.

The war in Syria has killed more than 250,000 people. Though with tens of thousands unaccounted for, some say the death toll may be as high as 400,000.