The main Syrian opposition group has asked for rebel fighters to be allowed to import weapons.
The plea came at a major international "Friends of Syria" conference being held in Tunisia to seek a breakthrough in the increasingly bitter conflict.
A declaration is expected later, calling on Syrian forces to declare a ceasefire and allow humanitarian access to the worst-hit areas.
Syrian state TV said the conference was a meeting of "symbols of colonialism".
Those attending, it said, were "historic enemies of the Arabs".
The US, Europe and Arab countries plan to challenge President Bashar al-Assad to provide humanitarian access within days, with the threat of fresh sanctions if he does not comply.
Around 70 nations, including the US, UK, France and Turkey, are attending the conference, organised by the Arab League.
But Russia and China, key allies of Syria which have blocked UN resolutions again Damascus, are not there.
A group of pro-Assad protesters forced their way into the grounds of the hotel where the conference is being held, Reuters news agency reported, but tight security prevented them getting into the building.
The leading opposition group, the Syrian National Council, said countries should be allowed to supply arms if Damascus refused to bow to outside pressure.
"If the regime fails to accept the terms of the political initiative outlined by the Arab League and end violence against citizens, the Friends of Syria should not constrain individual countries from aiding the Syrian opposition by means of military advisers, training and provision of arms to defend themselves."
The conference endorsed the council as a "credible" voice of opposition, while making clear it did not exclude other groups - thereby stopping short of declaring it a plausible government-in-waiting.
At least one other opposition group, the National Co-ordination Committee for Democratic Change (NCCDC), is boycotting the meeting, saying it excludes some voices and leaves open the idea of military intervention, AFP news agency reports.
Activists say more than 7,000 people have died in the 11-month uprising - more than 90 on Thursday alone - and concern is growing over the humanitarian situation, particularly in the besieged city of Homs.
The International Committee of the Red Cross (ICRC) appealed this week for a pause in fighting to allow aid to be taken in, but said it had received no response from Damascus.
The ICRC said it was becoming "more and more concerned over humanitarian needs that are increasing by the hour".
Spokesman Hicham Hassan told Reuters news agency: "It is crucial that our initiative is met with a positive and concrete reaction urgently."
US Secretary of State Hillary Clinton told the conference the Syrian government had "ignored every warning, squandered every opportunity and broken every agreement".
"If the Assad regime refuses to allow this life-saving aid to reach civilians, it will have ever-more blood on its hands,'' she said in opening remarks.
UK Foreign Secretary William Hague said "terrible crimes" were being committed in Homs.
"I think we have seen enough in the last few weeks to know that the Assad regime will go down in history as a criminal regime," he said.
French Foreign Minister Alain Juppe said the conference needed to exert the maximum pressure on the Syrian government and also on Russia, but insisted there was no military option on the table and France could not envisage such an option without an international mandate.
The BBC's Jonathan Marcus in Tunis says the conference is a means of getting around Russia and China, which have faced Western and Arab criticism for vetoing a UN Security Council resolution condemning Syria and Mr Assad.
Moscow and Beijing have said they want to see an end to the violence but that such action amounts to forced regime change.
On the eve of the conference, the UN and Arab League appointed Kofi Annan as their envoy to Syria.
Mr Annan, a former UN secretary general who has acted as a diplomatic troubleshooter in several long-running conflicts, said he hoped to "help bring an end to the violence and human rights abuses, and promote a peaceful solution to the Syrian crisis".
Diplomats attending the conference say the UN will call for preparations to start an Arab/UN peacekeeping force for Syria, to assist with the political transition after the violence ends, the BBC's Kim Ghattas reports from Tunis.
The civilian police force would be deployed only in a "permissive" environment, under Chapter 6 of the UN charter.
Diplomats said the efforts were designed to show a political transition was inevitable and that President Assad's rule was coming to an end.
The conference comes two days after two journalists - American Sunday Times reporter Marie Colvin and French photographer Remi Ochlik - died during shelling in Homs.
Two journalists wounded in the same attack have made internet appeals for medical help. Frenchwoman Edith Bouvier is being treated by Syrian medics but needs surgery which they are unable to perform. Paul Conroy, who is British, also asked for outside help to bring him to safety.