Rebels claimed to have launched a major assault on Syria's second city Aleppo as the UN refugee agency warned that up to 700,000 refugees could flee the country by year's end.
UN and Arab League leaders meanwhile expressed fears the country could become a "regional battleground."
"Tonight, Aleppo will be ours or we will be defeated," Abu Furat, a rebel commander, told AFP as several thousand fighters went on the offensive in the key northern city that has been a battleground since July.
The Syrian Observatory for Human Rights reported fierce combat in the early evening in the districts of Izaa and Saif al-Dawla in the southwest, involving "hundreds of rebel fighters."
The Britain-based group said 20 rockets had hit the central district of Sulamaniyeh, some of them striking an office of the political security service.
There were clashes in several districts, amid bombardment by government forces, it added.
In a video posted on YouTube in the name of the most important rebel unit, the Al-Tawhid Brigade, a man in civilian clothing clutching a walkie-talkie is seen giving instructions.
"Today the attack against Bashar's army began on all fronts," he says, referring to President Bashar al-Assad's forces. "God willing, today's battle in Aleppo will be decisive."
Amid the incessant rattle of rebel assault rifles and the occasional explosion of an army rocket, rebel fighters shouted "Allah akbar" (God is greatest).
"We shout to frighten the soldiers and to show them how many of us there are," said one rebel fighter.
Fighting was also reported in Homs, Hama, the coastal province of Latakia and the eastern province of Deir Ezzor.
Elsewhere, an oil pipeline in the northeastern province of Hasaka was sabotaged, according to the Observatory.
Wednesday was the deadliest day of the conflict, with more than 305 people killed nationwide, the Observatory said. That included 14 in twin bombings targeting the armed forces headquarters in central Damascus.
More than 30,000 people have been killed overall in violence since the March 2011 outbreak of the revolt against Assad, in a toll compiled by the Observatory.
Islamist rebel group Tajamo Ansar al-Islam said its men carried out Wednesday's bombings at the headquarters of the regime's armed forces, and that five of its fighters, including a suicide bomber, died during the assault.
Another group, Al-Nusra Front, also claimed the attack in a statement on jihadist forums.
The military said all senior commanders and other officers had escaped injury.
It was the biggest attack on the security apparatus since a July 18 suicide bombing killed four top regime officials, including defence minister General Daoud Rajha and Assad's brother-in-law, Assef Shawkat.
With the fighting escalating, the UN refugee agency warned that hundreds of thousands of refugees were streaming out of Syria, and that up to 700,000 could be in neighbouring countries by the end of the year.
Faced with the soaring need for aid, humanitarian agencies upped their call for funds to $487.9 million (379.2 million euros) to sustain operations until the end of the year.
At present, only $141.5 million in funding is available: just 29 percent of the overall sum requested.
The UN children's agency UNICEF also joined the appeal, saying more than 50 percent of the refugees were under 18 and one fifth under five.
The conflict dominated proceedings at the UN General Assembly in New York.
UN Secretary General Ban Ki-moon, Arab League leader Nabil al-Arabi and special envoy Lakhdar Brahimi meanwhile expressed their fears that Syria would become a "regional battleground" as the conflict worsened.
They met at UN headquarters to discuss "the appalling levels of violence" and efforts to help the special envoy, said UN spokesman Martin Nesirky.
"The three leaders warned against the risk of Syria turning into a regional battleground as violence intensifies.
"They were concerned that Syria will fall prey to actors whose agenda has nothing to do with Syria if violence continued," the spokesman added.
There was mounting Western pressure on Russia and China to ease their opposition to UN action against the Assad regime.
US Secretary of State Hillary Clinton denounced the UN's inability to act.
"The atrocities mount while the Security Council remains paralysed and I would urge that we try once again to find a path forward" for the council to try to end the violence," she said.
British Prime Minister David Cameron said the blood of children killed in the conflict had become "a terrible stain on the reputation of this United Nations."
Russian President Vladimir Putin, whose country stands accused of blocking UN action, accused the West of pursuing policies that had destabilised Arab countries.
"They have already created a situation of chaos in many territories and are now continuing the same policy in other countries -- including Syria," news agencies quoted him as saying.