More than 300 civilians were killed in two weeks of fighting in the divided city before the truce took hold on Thursday.
Thirteen military advisers with the Iranian Revolutionary Guards have been killed in Syria in recent days and 21 others wounded, Iranian media reported on Saturday.
It was Iran's biggest loss of forces within such a short time, based on official figures. The names of those killed and when their remains will be repatriated will be announced later, the Guards said.
All were from Iran's northern province of Mazandaran, Hossein Ali Rezayi, a Guards spokesman in the region, told the ISNA and Fars news agencies.
The deaths and injuries occurred in Khan Tuman village some 10 kilometres (six miles) southwest of the battleground city of Aleppo, the official IRNA news agency reported a Guards statement as saying.
Pro-regime troops had driven terrorists out of Khan Tuman in December, but on Friday a monitor reported more than 70 killed in fighting between regime forces and Al Qaeda-affiliated terrorists and their allies south of Aleppo.
Al-Nusra Front and allied Islamists seized Khan Tuman and surrounding villages after less than 24 hours of clashes, the Syrian Observatory for Human Rights said.
Around 30 pro-regime troops were killed in the battle, said the Britain-based Observatory which relies on a network of sources in Syria.
Russia said late 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, in regime air strikes on its opposition-held east and rebel shelling of the regime-controlled west.
Iran is Syria's main regional ally, sending financial and military aid, including military advisers and volunteer forces from Iran, Afghanistan, Iraq and Pakistan, to prop up President Bashar al-Assad's regime.
Dozens of Iranian "advisers" have been 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.
"The Islamic republic of Iran has employed all of its capacity for the fight against terrorists committing crimes against oppressed nations in the region," state television news agency IRIB quoted him as saying.
Assad told Velayati Syria was "very hopeful of victory in this unbalanced war, thanks to Iran's selfless support", IRIB reported.
Tehran late on Saturday condemned the capture of Khan Tuman as a breach of the ceasefire.
"The joint action by terrorists and irresponsible armed groups called moderates, in abusing the ceasefire atmosphere in Syria, proves that these movements want military measures to continue," said deputy foreign minister Hossein Amir Abdollahian.
"They have no belief in a political solution," he said in the statement carried by IRNA.