Outside powers need to act fast to protect Syrians because foreign military backing for the government is "changing the equation" of the war, Qatar's foreign minister said on Wednesday.
For 14 days, a Russian-backed Syrian government offensive has been underway to capture eastern Aleppo and crush the last urban stronghold of a revolt against Syrian President Bashar al-Assad that began in 2011.
The collapse of the latest Syria ceasefire has heightened the possibility that Gulf states including Saudi Arabia and Qatar - backers of Syrian rebels - might arm the opposition with shoulder-fired missiles to defend themselves against Syrian and Russian warplanes, U.S. officials have said.
Qatar's foreign minister, Sheikh Mohammed bin Abdulrahman al-Thani, a member of the royal family, told a news conference in Singapore on Wednesday that the Friends of Syria group - an alliance of mainly Western and Gulf Arab countries who oppose Assad - had since February "stopped and minimized" its supply of weapons to rebels.
"On the other hand, the regime is continuing to receive supply from its allies, and that is what is changing the equation right now, and we hope that we can do something to protect them," al-Thani said in answer to a question.
"We are very much concerned that the international community doesn't have a Plan B for the Syrian conflict and the bombing and the shelling of the Syrians will continue. We need to act very fast to protect the Syrian people."
The United States maintains that negotiations are the only way to end the violence in Syria.
U.S. Secretary of State John Kerry, who announced on Monday that Washington was suspending talks with Moscow due to Russia's role in the offensive, has said peace efforts must carry on.
But frustration with Washington has intensified, raising the possibility that Gulf allies or Turkey will no longer continue to follow the U.S. lead or will turn a blind eye to wealthy individuals looking to supply man-portable air defence systems, or MANPADS, to opposition groups.
Western nations, mindful of how weapons spread from Libya after its Western-backed revolt in 2011 to unstable nations such as Mali, say arming rebels is risky because it is hard to tell militants from moderates in a disorganized array.
Gulf Arabs say they know Islamist armed groups better and blame disorganization on a lack of outside support and training.