Syrian rebels on Saturday shelled government-held districts in the northern city of Aleppo, casting doubts on the future of a temporary local ceasefire announced by the government's Russian ally.
Tens of shells fell on government-held western Aleppo, wounding one person and causing heavy material damage, state news agency SANA said.
The Britain-based Syrian Observatory for Human Rights, which monitors violence across the country, confirmed that Islamist rebels, who largely control the eastern half of the divided city, had shelled western Aleppo.
Shortly before midnight on Friday, Russia's Defence Ministry announced that a temporary truce in Aleppo had been extended for three more days, starting at 0100 Saturday (2200 GMT Friday).
"This is a clear violation of the truce," said Rami Abdel-Rahman, head of the Observatory.
Violence has simmered since April 22 in Aleppo, with at least 286 civilians killed in reciprocal attacks by regime forces and rebels, the Observatory said in an updated death toll.
The dead included 57 children and 38 women.
The surge in fighting and civilian deaths has prompted international diplomatic efforts to shore up an earlier US-Russian-brokered ceasefire that took effect in most parts of Syria in late February.
That truce excluded the Islamic State extremist militia and the al-Qaeda-linked al-Nusra Front.
Al-Nusra has spearheaded recent rebel advances to the south of the city, including the capture on Friday of the town of Khan Touman which, according to the Observatory, formed a major defensive line for government forces in the area.
On Saturday, al-Assad accused unnamed Western and regional powers of supporting "terrorism" in his country.
"Several Western and regional countries, which have stoked terrorism in Syria in the past years, continue to support terrorists secretly and in public," al-Assad said, according to SANA.
"This support continues despite the efforts being exerted to cease hostilities and reach a political solution in Syria," he said at a meeting in Damascus with Ali Akbar Velayati, an advisor to Iran's supreme leader.
Al-Assad usually uses the term "terrorists" to refer to the opposition rebels fighting to oust him.
Iran has been a staunch backer of al-Assad since the pro-democracy uprising started against his regime in 2011.
At least 13 members of Iran's Revolutionary Guard have been killed in Aleppo, a spokesman for the IRGC told the ISNA news agency on Saturday.
The spokesman said 21 were injured, but didn't provide further information.
Iran maintains that it has no troops in Syria and that its Revolutionary Guards merely act as military advisers to the Syrian army.
Opposition sources charge that most of those fighting on the government's side in northern Syria are in fact members of pro-Iranian Shiite militias from Lebanon, Iraq and other countries.
Al-Assad's army has been worn down by five years of conflict.
Several Western and regional powers, backing the Syrian rebels, have repeatedly called for al-Assad to step down in order to end the conflict.
Meanwhile, at least six civilians and four members of the Islamic State extremist group were killed in US-led airstrikes north of Aleppo over a 24-hour period, the Observatory said.
The strikes targeted the town of Dabiq, a place that has mystical symbolism for the extremist group, and the town of Ihtimalat, according to the Britain-based watchdog,
A high-ranking member of the jihadist group was among those killed, the Observatory said.
Western- and Turkey-backed rebels have been attempting to capture Islamic State's last strip of territory along the border, but in recent weeks the extremists have reversed earlier rebel gains.
Government and Kurdish-led forces, the former backed by Russia and the latter by the US, have also been attempting to push into Islamic State's territory in the area.