Thirteen military advisers from the Iranian Revolutionary Guards have been killed in Syria in recent days, and 21 others injured, Iranian media reported on Saturday.

This marks Iran's biggest recent loss of senior military figures. The names of those killed and when their remains will be repatriated will be announced later, but all were from the province of Mazandaran in northern Iran, said Hossein Ali Rezayi, a Guards spokesman, to ISNA and Fars news agencies.

The deaths and injuries occurred in the village of Khan Tuman, 10 kilometers (6 miles) southwest of the battleground city of Aleppo, the official IRNA news agency reported a Guards statement as saying.

On Friday, a monitor reported more than 70 killed in fighting between regime forces and opposition groups south of Aleppo. The al-Qaida-linked Jubhat al-Nusra, or the Nusra Front, seized Khan Tuman and surrounding villages with allied groups after less than 24 hours of clashes, according to the Syrian Observatory for Human Rights.

Around 30 pro-regime troops were killed in the battle, said the U.K.-based Observatory, which relies on a network of sources in Syria. Russia said late on Friday that a temporary truce in Aleppo had been extended for 72 hours "in order to prevent the situation from worsening."

More than 300 civilians were killed in two weeks of fighting in the divided city before the truce took hold on Thursday, by regime air strikes on the opposition-held east and rebel shelling of the regime-controlled west.

Iran is main regional ally of the Syrian regime, sending financial and military aid, including military advisers and volunteer forces from Iran, Afghanistan, Iraq and Pakistan, to prop up the regime of Bashar al-Assad. Dozens of Iranian "advisers" were killed in Syria since late 2015, including Revolutionary Guards commanders.

Saturday's news came as Ali Akbar Velayati, a top adviser to supreme leader Ayatollah Ali Khamenei, met Assad in Damascus and assured him of Tehran's support. Iranian state television news agency IRIB quoted Velayati as saying, "The Islamic Republic of Iran has employed all of its capacity for the fight against terrorists committing crimes against oppressed nations in the region." IRIB reported that Assad told Velayati that Syria was "very hopeful of victory in this unbalanced war, thanks to Iran's selfless support."