Turkey's prime minister has said neighbouring Syria is a "terrorist state" that is massacring its own people, just hours after the new UN-Arab League envoy to the country called the death toll in the uprising "staggering".
According to opposition activists, more than 26,000 people have died in the last 17 months, while Unicef says last week was the deadliest since the conflict began with about 1,600 people killed.
Turkey's Recep Tayyip Erdogan told a meeting of his AKP party that President Bashar al Assad was "up to his neck in blood".
"Syria is not an ordinary country to us. We do not have the luxury to remain indifferent to what's happening there," he said.
Mr Erdogan has repeatedly condemned the Syrian regime in recent months, despite Turkey enjoying good relations with Syria for many years, but the Turkish prime minister was also critical of other nations for their failure to act.
"The massacres in Syria that gain strength from the international community's indifference are continuing to increase," he said.
New UN-Arab League Lakhdar Brahimi has said he will go to Damascus "in a few days" in an attempt to mediate in the conflict but has also warned that the situation is "deteriorating steadily".
"The death toll is staggering, the destruction is reaching catastrophic proportions and the suffering of the people is immense," he said in a brief speech to the UN assembly.
"The future of Syria will be built by its people and none other," the 78-year-old diplomat added. "The support of the international community is indispensable and very urgent. It will only be effective if all pull in the same direction."
Former envoy Kofi Annan quit his six-month bid to end the war complaining at the lack of international support to make Mr Assad carry out his peace plan.
Meanwhile, Egypt urged Mr Assad to "takes lessons from recent history" and step aside.
"The Syrian people have made their voice clear," Mohamed Morsi, the Egyptian president, said. "You will not be around for long."
Mr Morsi, who was elected as Egypt's first Islamist leader in June, urged Arab nations to act to solve the crisis.
"The Syrian blood that is being shed day and night, we are responsible for this," he said. "We cannot sleep while Syrian blood is being shed."
Amid the warnings over the escalating violence, rebel forces reportedly shot down a Syrian military fighter jet in northern Idlib province as it was taking off rom an air base.
"They brought it down as it was taking off from the airport using 14.5 millimetre anti-aircraft machineguns," Abu Majad, a spokesman from the rebel Ahrar al Sham (Free Men of Syria) brigade, said.
"The airport has been surrounded by the Ahrar al Sham brigade and the Syrian Martyrs Brigade," he said. "Due to communication difficulties I am not sure what the situation is at the moment."
There was no immediate comment on the rebel claims from the government, but rebel forces have stepped up their attacks on air force targets in recent weeks in an attempt to halt attacks on their targets by military aircraft.
Hamdan military airport, in the eastern province of Deir Ezzor near the Iraqi border, was also under attack on Wednesday, according to the opposition.
"Fighting has been going on for hours inside Hamdan airport between soldiers and rebels, who have taken over large sections of the site," Syrian Observatory for Human Rights director Rami Abdel Rahman said.
UN leader Ban Ki-moon has accused countries who send arms to Syria of spreading "misery" and called for greater international efforts to end the war and ease a mounting humanitarian crisis.
"Those who provide arms to either side are only contributing to further misery - and the risk of unintended consequences as the fighting intensifies and spreads," he said.
Mr Ban did not name any country, but Russia is President Bashar al Assad's main arms supplier while UN officials say Iran has made arms deliveries to his forces.
The Syrian government accuses Saudi Arabia, Qatar and Turkey of arming the opposition. The US and Great Britain have said they give non-lethal assistance to the opposition.