Qatar has called for an emergency meeting of the Arab League and urged the international community to "assume its responsibilities" as the Syrian government keeps up its air offensive on the northern city of Aleppo.
Government warplanes and helicopter gunships launched new air strikes on Saturday on Aleppo's rebel-held neighbourhoods, killing at least eight people, officials said.
In the rebel-held east, dozens of civilians left the Bustan al-Qasr district on the ninth straight day of the Syrian military onslaught, an AFP news agency correspondent said.
The few people out on the streets watched the sky anxiously for government warplanes, running for shelter when one launched a new raid.
In Pictures: Syria's civil war - Inside the bomb shelter
Responding to the Aleppo assault, the Syrian opposition accused the government of committing war crimes on civilians.
Speaking after a meeting in Istanbul in Turkey of the Syrian National Coalition, Anas al-Abdeh, the group's leader, said: "Aleppo has been reeling under intense, ferocious bombardment. It is a systematic campaign. Therefore our meeting today focused on the situation in the besieged area, including Aleppo city.
"It is clear that the regime's uninterrupted shelling and air strikes amount to war crimes and crimes against humanity.
"They attacked medical facilities, residential areas and a bakery catering for more than 300 families."
Conflict's worst fighting
Once Syria's economic hub, Aleppo and its surrounding countryside have suffered some of the worst fighting in a conflict that has killed more than 270,000 people and displaced millions.
The city has been divided into rebel and government-controlled zones.
At least 246 civilians have died in shelling, rocket fire and air strikes on both sides of the city since April 22, the Syrian Observatory for Human Rights monitor said.
The Britain-based monitoring group, which relies on a network of sources on the ground, reported on Saturday 28 air strikes on eastern neighbourhoods.
But in its daily report on Syria, Russia's defence ministry said it had recorded only "three ceasefire violations in the city of Aleppo", blaming them all on the rebels.
Syria's SANA state news agency said shelling of western government-held neighbourhoods killed three civilians, including a child, and blamed al-Nusra Front and its allies.
Speaking to Al Jazeera, Louay Safi, a former Syrian National Coalition leader, said: "There is an all-out war on Aleppo.
"The intensity of bombing is very high. They are hitting water facilities, public facilities ... the regime hasn't spared anybody.
"A large number of people have been killed in Aleppo and many of those who have been hurt are elderly and children. They are targeting residential areas using barrel bombs which are indiscriminate."
A pro-government newspaper said on Thursday the army was preparing an offensive to recapture all of Aleppo and the surrounding province.
Some families have fled to safer districts, while others have left by the dangerous Castello Road, the only route out of east Aleppo.
Hospitals have also been bombed: four medical facilities were hit on Friday on both sides of the frontline, the International Committee of the Red Cross said.
A raid on Wednesday hit a hospital supported by Doctors Without Borders and the Red Cross as well as nearby housing, killing 30 people.
Aleppo was left out of a new temporary US-Russian brokered truce in the government stronghold of Latakia as well as Damascus and the nearby rebel-held Eastern Ghouta.
Fighting halted at 1am on Saturday in a "freeze" that held for 24 hours in Damascus and Eastern Ghouta, and was set to last for another 48 hours in Latakia.
On the diplomatic front, John Kerry, the US secretary of state, is due to travel on Sunday to Geneva in a show of support for the temporary truce and will meet the UN envoy to Syria and the Jordanian and Saudi foreign ministers.
The state department said his talks will focus on "efforts to reaffirm the cessation of hostilities nationwide in Syria".
A new round of UN-backed peace talks is set to start on May 10 in the Swiss city.
The US has appealed to Russia to keep President Bashar al-Assad's government in check.
Russia, however, has said that it will not ask Syria to halt the air raids on Aleppo.
"No, we are not going to put pressure on [Syria] because one must understand that the situation in Aleppo is part of this fight against the terrorist threat," Gennady Gatilov, deputy foreign minister, said.
|Aleppo, Syria's largest city and former commercial hub, is not part of a temporary, partial ceasefire effective elsewhere [Al Jazeera]
The Qatari request to the Arab League came in the form of a memo seeking discussion on "the dangerous escalation seen in Aleppo where civilians are subjected to massacres at the hands of the forces of the Syrian regime, which has led to the deaths and injuries of hundreds".
Qatar's state news agency QNA said Mohammed bin Jassim Al Thani, the foreign minister, made phone calls to his Turkish, French and British counterparts to discuss the humanitarian situation in Aleppo "in light of the brutal shelling and air strikes of the regime forces in Syria targeting civilians and residential areas".
Saying that the assault was in defiance of all international charters and principles, he called for the international community to "to stop the ongoing massacre and protect the Syrian people".
A similar message was conveyed by Sultan bin Saad al-Meraikhi, Qatar's assistant foreign minister, in a phone call to Mikhail Bogdanov, Russia's deputy foreign minister, QNA reported.
Saudi Arabia has also called on the international community and allies of Assad who had committed themselves to the "cessation of hostilities" to take necessary measures to stop "the attacks and crimes against the people of Syria".
Source: Al Jazeera and agencies