The Syrian army was on Monday reported to be sending reinforcements to Aleppo, where renewed fighting is threatening a fragile truce in the run-up to the next round of peace talks.
Underlining the conflict's regional dimensions, Iranian media announced the first deaths of members of its regular army in Syria, a week after Tehran said army commandos had been deployed in support of Damascus. Iran's military support has so far mostly been provided by the elite Revolutionary Guard Corps.
An eruption of fighting near the ancient city of Aleppo in the last two weeks marks the most serious challenge yet to a "cessation of hostilities" brokered by the United States and Russia with the aim of facilitating peace talks.
Pointing to the frayed state of the truce, Foreign Minister Walid al-Moualem told U.N. envoy Staffan de Mistura, who is visiting Damascus, that Turkey and Saudi Arabia were behind violations of the deal.
He said they had ordered insurgents to stage attacks aimed at foiling planned Geneva talks. There was no immediate response from Saudi Arabia and Turkey.
The two nations have backed the rebellion against President Bashar al-Assad, providing insurgents with arms and money. Assad is supported militarily by both Iran and Russia.
The U.N.-sponsored talks, which resume on Wednesday, aim to end a five-year-old conflict which has killed more than 250,000 people, created the world's worst refugee crisis and allowed for the rise of Islamic State. The first round made little progress, with no sign of compromise over the key issue of Assad's future.
Underlining Assad's confidence, the Syrian government is due to hold parliamentary elections in state-held parts of the country on Wednesday. The opposition has called the vote a sham.
Fighting for Aleppo
The fighting near Aleppo has focused around a cluster of towns along the main road to the south.
Rebels say the army has also intensified bombing, and Russian warplanes have resumed air strikes in the area.
The army has accused rebels of taking part in attacks by the Nusra Front, an al Qaeda-linked group, which along with Islamic State was not included in the truce agreement.
Russia said on Monday that Nusra was massing around Aleppo ahead of a major offensive.
Syria's Prime Minister Wael al-Halaki was quoted on Sunday as saying the government, backed by Russia's air force, was planning an operation to retake Aleppo, but the Russian defense ministry said there were no plans to storm the city.
Local media on both sides reported a large build-up of troops and equipment by the Syrian army and its allies around Aleppo, with the pro-Damascus al-Mayadeen TV reporting it had seen tanks and rocket launchers heading towards the city.
The government and its allies have mounted major operations against insurgents to the north and south of Aleppo in the six months since Russia began air strikes in support of Assad and cut the most direct supply route to Turkey earlier this year.
But rebels still hold territory in and around the city, including its western approaches.
The two fiercest fronts in fighting around Aleppo in recent days have been in the towns of Telat al-Eis, Zitan, Zirba and Khan Touman on the main highway south to Damascus, and around the Handarat camp on a main road running north to Turkey.
The Aleppo front is one of the areas where Iranian Revolutionary Guards and Lebanon's Hezbollah have deployed in support of the army.
The Iranian Tasnim news agency said four soldiers in Iran's regular army had been killed in Syria, without saying when or where. "Four of the first military advisers of the Islamic Republic's army ... were killed in Syria by takfiri groups," it said, referring to hardline Sunni Islamists.
Trying to protect ceasefire
Both Damascus and the opposition's High Negotiations Committee have held the other to blame for breaches in the truce, which came into effect on Feb. 27.
De Mistura was in Damascus for meetings with senior government officials before traveling on to Iran in an attempt to revive the peace talks after negotiations in March failed to make much progress.
The next round will focus on a political transition, de Mistura said. Moualem said the government would be ready to take part.
Meanwhile, fighting also erupted between rebels and Islamic State on Monday, as the group reclaimed the town of al-Rai near the border with Turkey, about 50km (30 miles) from Aleppo, only days after it fell to the Turkish-backed rebels.