BEIRUT — With tanks and artillery, the Syrian Army pounded opposition strongholds in Aleppo on Saturday, stepping up its barrage on a city that for days has been steeling for an assault, residents and activists said.
It was not clear whether the attack, which activists said was focused on the southwestern Salaheddiin neighborhood, was a limited foray by government troops or the beginning of a broader campaign.
Activists and residents said that opposition fighters had at least partially repelled the assault, killing soldiers and destroying several tanks, but those claims could not be immediately verified.
The clashes came after days of warnings from the international community about the human toll in Aleppo, Syria’s largest city and its commercial center, as President Bashar Assad’s forces massed on the outskirts of the city. For days, rebel fighters have been pouring into Salaheddiin and other neighborhoods in Aleppo, which had remained quiet for much of the uprising that started in March 2011.
On Saturday, Russia, Assad’s most important ally, joined the chorus, warning of tragedy as it chastised the rebels’ foreign backers for failing to pressure the opposition to end the violence.
In Aleppo, a Salaheddiin resident named Mohammed, who did not want to give his last name for fear of retaliation, said the government’s assault began at dawn as helicopters flew overhead and a warplane circled. The shelling began soon afterward and lasted for hours, far heavier and fired from closer to the city than in recent days.
‘‘Huge sounds of explosions,’’ he said by telephone. ‘‘Three or four bombs at once. Those who didn’t leave are hiding in lower floors. Others fled to schools or mosques.’’
A BBC reporter in Aleppo, Ian Pannell, spoke in a broadcast about skirmishes and explosions and said that Salaheddiin had come under a ‘‘sustained attack.’’ Activist groups reported more than 20 deaths in the fighting, including civilians, opposition fighters, and government soldiers.
A video posted on the Internet by activists showed scenes of chaotic urban combat, with militiamen firing wildly around corners as families fled the other way. Another video purportedly depicting the aftermath of clashes showed bodies in a street and a smoldering tank. A narrator asserted that they were government soldiers, calling them ‘‘Bashar’s dogs.’’
A dentist in Aleppo who was supervising an effort to shelter people displaced by the fighting said 900 were crammed into five schools in Salaheddiin and another neighborhood.
There was not enough food to feed them all, she said, adding, ‘‘The shelling around us never stops.’’
On Saturday, diplomatic skirmishing over how to stop the conflict continued outside Syria.
Sergey V. Lavrov, the Russian foreign minister, echoed concerns about violence in Aleppo and called on Assad’s government to ‘‘make the first moves’’ in ceasing military action. But he also blamed Western countries and some of Syria’s neighbors for not putting enough pressure on the armed opposition to stop fighting.
Speaking in Sochi, Russia, Lavrov said that those countries ‘‘encourage, support, and direct the armed fight against the regime.’’
Although he did not name any countries, Turkey, Saudi Arabia, and Qatar have been helping the Syrian rebels obtain weapons and American officials say US intelligence officers are operating in southern Turkey to help decide which groups receive the arms.
Russia said this month that it would halt any weapons shipments to the government of Assad. On Saturday, though, the Russian Foreign Ministry said it would not cooperate with a European Union effort to block such shipments by searching ships suspected of carrying weapons to Syria.
A ministry spokesman said Russia considered the plan to inspect ships a violation of other countries’ sovereignty.
In comments to the Interfax news agency, Lavrov dismissed the notion that Russia would grant Assad asylum, saying it was a rumor started to make Russia look bad.
“There is no such agreement, we are not even thinking about this matter,’’ he said.