With tanks and artillery, the Syrian army pounded opposition strongholds in Aleppo today, 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 neighbourhood, 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 centre, as president Bashar Assad's forces massed on the outskirts of the city.
Today, diplomatic skirmishing over how to stop the conflict continued outside Syria.
Russian foreign minister Sergey Lavrov echoed concerns about violence in Aleppo, and called on Dr Assad's government to "make the first moves" in ceasing military action.
But he also blamed Western countries and some of Syria's neighbours for not putting enough pressure on the armed opposition to stop fighting.
Mr 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.
US officials have said that the United States would provide more communications training and equipment to help improve the rebels' effectiveness.
Speaking in Sochi, Russia, Mr Lavrov added: "It is unrealistic to expect the government to admit that it was wrong and allow itself to be overthrown. And not because we are clinging to that regime, but because such a scheme does not work."
Russia, one of Syria's most important foreign supporters, said this month that it would halt any weapons shipments to the government of Dr Assad.
However, Russia's foreign ministry said it would not co-operate 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 Aleppo, a resident of the Salaheddiin neighborhood named Mohammed, who did not want to give his last name for fear of government retaliation, said the government's assault began at dawn, as helicopters flew overhead and a warplane circled.
The shelling began soon after that and lasted for hours, far heavier and 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."
Rebel fighters have been pouring into Salaheddiin and other neighborhoods in Aleppo, which had remained quiet for much of the uprising that started last year.
The Syrian Observatory for Human Rights, based in Britain, said at least two opposition fighters and 10 government soldiers had been killed in the early fighting in Salaheddiin and nearby neighbourhoods.
Journalists in the city reported clashes and said they saw injured opposition fighters. A video posted on the internet by activists showed scenes of confused urban combat, with militiamen firing wildly around corners, as families fled down the street.
Another video purportedly depicting the aftermath of clashes showed bodies in a street and a smoldering tank. A narrator asserts that they are government soldiers, calling them "Bashar's dogs." One of the men, wearing fatigues and lying on his side, appears to be clutching a machine gun.
New York Times