The head of the opposition Syrian National Council (SNC) has called for foreign states to arm rebel fighters.
Abdulbaset Sayda was speaking as Syrian forces continued their assault on rebel-held areas of the city of Aleppo.
Mr Sayda also said that President Bashar al-Assad should be tried for "massacres" rather than be offered asylum.
Western nations have warned of a potential bloodbath in Aleppo, Syria's most populous city.
"We want weapons that would stop tanks and jet fighters. That is what we want," AFP quoted Mr Sayda as saying at a news conference in Abu Dhabi.
He urged Arab "brothers and friends to support the Free [Syrian] Army".
Rebels have so far not received any overt foreign military support.
In April, Gulf states agreed to pay "the salaries" of Free Syrian Army fighters. Critics said the funds could be used to buy weapons on the black market.
Mr Sayda said that the "Yemeni example" in which President Ali Abdullah Saleh was granted amnesty in return for stepping down "can't be applied in Syria".
"There are massacres being committed. We believe Bashar al-Assad should be tried. He is a criminal and should not be given shelter," he added.
Mr Sayda said the SNC - which is in exile - would discuss plans for transitional government with rebel groups involved in the fighting.
"We are studying the idea (of transitional government) and we will contact all forces on the ground in Syria," he said.
He said the leader of such a government should be an "honest and patriotic person... committed to the objectives of the Syrian revolution since its beginning".Annan 'concern'
UN-African Union envoy Kofi Annan said on Saturday he was "concerned" about reports of the concentration of troops and heavy weapons around Aleppo "in anticipation of an imminent battle".
"The escalation of the military build-up... is further evidence of the need for the international community to come together to persuade the parties that only a political transition, leading to a political settlement, will resolve this crisis," he said.
Activists said Syrian tanks moved in on south-western districts of Aleppo city early on Saturday.
Rebel-held areas were bombarded throughout the day, with military aircraft flying over at low altitudes, they said.
Violent clashes were reported around the Salah al-Din and Hamdanieh quarters near the centre.
However, by the end of the day it appeared that the rebels had largely held their ground.
They are believed to control between a third and a half of all neighbourhoods in the city of three million people, especially a cluster in the north-east around Sakhour and other areas in the south-west.
The BBC's Ian Pannell, who has been in Aleppo, has seen fierce battles, with a number of rebel fighters killed.
He says the rebels are upbeat but vastly outgunned and outmanned and that civilians have packed on to lorries as they try to flee the city.'Terrorist den'
The UK-based Syrian Observatory for Human Rights said 29 people were killed in Aleppo on Saturday - among 168 to die across the country throughout the day. The figures cannot be verified.
The total number of people killed since the Syrian anti-government uprising began in March 2011 now stands at more than 20,000, the Observatory says.
Syrian state television said that rebels, having failed to hold areas of the capital Damascus, were now trying to turn Aleppo into "a den for their terrorism".
The state-controlled al-Watan newspaper proclaimed the fight for Aleppo as "the mother of all battles."
On Saturday, Russian Foreign Minister Sergei Lavrov said a tragedy was brewing in Aleppo, but it was unrealistic to expect the Syrian government to accept the presence of an armed opposition.
He added that Russia - an ally of Syria - was not considering giving refuge to President Assad.
The latest fighting comes after two weeks in which rebels have made significant gains.
An attack at Syrian security headquarters in Damascus on 18 July killed four senior officials, including the defence minister and President Assad's brother-in-law.
Then the Free Syrian Army (FSA) took control of several parts of Damascus before being driven out by a government counter-offensive.
The rebels also seized several border crossings with Turkey and Iraq.
Meanwhile, neighbouring Jordan is opening its first official refugee camp for Syrians fleeing the violence.
The UN refugee agency (UNHCR) says the new camp at Za'atari will initially house 10,000 people but could eventually give refuge to more than 100,000.
The UN says 150,00 Syrians have already entered Jordan.
The BBC's Lyse Doucet in Jordan says the new camp will ease pressure on existing transit camps where overcrowding has been causing tension between refugees and with local communities.