Foreign ministers of the Friends of Syria group, who are meeting in Qatar, have agreed to provide urgent support to rebels who are fighting President Bashar al-Assad.
Qatar's PM Sheikh Hamad bin Jassim bin Jaber al-Thani said "providing arms may be the only means of achieving peace".
The group also condemned the Syrian government for its use of Iranian and Hezbollah fighters.
More than 90,000 people have died in more than two years of conflict.
The Syrian government says it is fighting foreign-backed "terrorists".
'Balance on the ground'
The Friends of Syria group includes the US, Britain, France and Germany as well as Saudi Arabia, Turkey, Egypt and Jordan.
Its joint statement said the members had agreed to "provide urgently all necessary materiel and equipment to the opposition on the ground, each country in its own way in order to enable them to counter brutal attacks by the regime and its allies".
Support would be channelled through the Western-backed rebel military command.
The group also called on the immediate withdrawal of Lebanese Hezbollah and Iranian fighters from Syria.
The meeting in Qatar's capital, Doha, comes a week after the US announced it would provide Syrian rebels with "direct military aid".
That decision followed what the US said was evidence of small-scale chemical weapon attacks by the government.
The Friends of Syria group was set up in response to moves by Russia and China to block UN resolutions on Syria.
On Saturday, the Qatari PM said that "failure to reach an agreement in the UN Security Council will not stop us moving forward".
He said that moral support for the rebels was not sufficient, adding: "A balance must be achieved on the ground so the regime can accept negotiations."
The group's statement did not mention the specifics of the support that would be provided.
US Secretary of State John Kerry said again that the Syrian government had crossed a line in using chemical weapons
But he insisted that the decision to provide military support to the rebels was "not to seek a military solution" but to give the rebels more power in negotiating an end to the conflict.
Mr Kerry said the group was still pushing for a peace conference in Geneva between the two opposing sides in Syria.
But he said Mr Assad had responded to the calls for a conference by bringing in Iranian and Hezbollah fighters to confront the rebels.
"That is a very, very dangerous development,'' he said. "Hezbollah is a proxy for Iran... Hezbollah in addition to that is a terrorist organisation."
UK Foreign Secretary William Hague said: "We have been talking about how we can help the opposition, how we can help save lives. Different countries will have different ways of doing that.
"We have to cooperate and we have to coordinate as much as possible to try to save the lives of innocent people in Syria."
He said again that the UK government had taken no decision to arm the rebels.
The rebels have suffered a series of military setbacks recently.
But on Friday, they said they had received new weapons that could lead to "changes" in the civil war.
A spokesman for the rebel Free Syrian Army said they had not come from the US.
"We've received quantities of new types of weapons, including some that we asked for and that we believe will change the course of the battle on the ground," FSA spokesman Louay Muqdad told AFP news agency.
On the ground in Syria on Saturday, government forces were reported to have stepped up their attacks on rebel positions north of the capital, Damascus.
The pro-opposition Syrian Observatory for Human Rights reported activists as saying there had been heavy shelling in a number of districts.
But the rebels said they had declared a new offensive of their own in the northern city Aleppo, the last major rebel stronghold.
They are attempting to capture government-held areas in the west of the city, Syria's largest.