Iran told visiting UN envoy Staffan de Mistura of its alarm Tuesday about what it said were ceasefire breaches in Syria on the eve of fresh peace negotiations in Geneva.
De Mistura's visit to Tehran – a key backer of President Bashar al-Assad – coincided with an announcement of more Iranian military deaths in Syria.
Four soldiers were killed in clashes with several thousand Al-Nusra Front fighters south of Aleppo, said General Ahmad Reza Pourdastan, commander of the Iranian army's ground forces.
"Our fighters have shown courage and four of them have fallen as martyrs," he told state television, saying they had killed 200 Al-Nusra members and destroyed the group's tanks and transport.
He nevertheless insisted that the Iranians were in the area as "advisers" in support of Assad's forces.
At least 220 Iranian fighters have been killed in Syria over the past six month alone.
Jihadists such as those from Al-Nusra and the Islamic State group are excluded from the ceasefire, but in some areas the Al Qaeda jihadists are allied with rebel forces meant to be covered by the truce – highlighting the delicate nature of the ceasefire deal.
De Mistura held talks with Iran's deputy foreign minister Hossein Amir Abdollahian ahead of the latest effort to end five years of fighting.
The Syrian conflict has killed more than 270,000 people and forced millions to flee their homes, creating a refugee crisis in Europe.
Iran and Russia have been Assad's top supporters in the war, with Tehran providing economic and military support that has propped up his rule.
Some of that aid has included thousands of fighters from Iran itself – particularly the Revolutionary Guards, Iranian army and Basij paramilitary – and from Hezbollah in Lebanon, as well as thousands of Shia jihadist "volunteers" from Iraq and Afghanistan.
But despite much evidence of Iranian soldiers fighting – and even commanding forces – on the front lines, Tehran insists they are just in Syria as "advisers."
De Mistura's trip came as the ceasefire, brokered by the United States and Russia and in place since February 27, was threatening to fall apart.
"We explained to Mr de Mistura our concerns after an increase in recent days of military action from irresponsible armed groups and the increase in violations of the ceasefire," state television quoted Abdollahian as saying.
"This is disturbing and may interfere with the political process."
De Mistura, quoted by official IRNA news agency, said his talks with Iranian officials were important "because their position and ideas are effective and we use their recommendations."
Iran frequently describes the rebel forces, who have received various levels of Western backing as well as arms from Arab Gulf states seeking Assad's overthrow, as "terrorists."
The UN Security Council passed a resolution in December that led to the peace talks. A framework called for Syrian elections to be held 18 months after a transitional government is agreed.
The fate of Assad is a major stumbling block, however.
While the Syrian opposition insists Assad can play no role in a transitional government, both the Damascus regime and Iran say voters should decide his fate.
AFP contributed to this report.