The Syrian branch of the Muslim Brotherhood called on Sunday for the United States and Europe to deliver the rebels promised military support after the opposition's National Coalition voted in a new president and ended a months-long leadership vacuum.
The United States has signalled it is ready to send arms to the rebels, in addition to humanitarian and non-lethal military aid already in the millions of dollars. Its Gulf allies Qatar and Saudi Arabia are already sending in arms.
But there have been no confirmed reports of shipments reaching Syria from its hesitant Western backers.
"We feel abandoned and disappointed that the United States and Europe have backed down from their position regarding arming the Free Syrian Army and we call on the international community to fulfil its obligations," said a statement posted by the Brotherhood on Twitter.
The opposition has been calling for more advanced weaponry since President Bashar al-Assad's forces launched a new offensive in central Syria with the help of the Lebanese militant group Hezbollah.
His forces are hammering the central city of Homs and have encircled rebel strongholds near the capital Damascus.
Western powers are still hesitant about sending arms, partially due to the fractious politics of the opposition's political umbrella group abroad, the Syrian National Council. It went months without being able to create enough agreement within its ranks to hold elections for a new leader, after the former president, Moaz Alkhatib, resigned.
They are also concerned about the growing reach of radical rebel Islamist groups in Syria, some of them linked to al Qaeda, who could end up with Western weapons.
Melhem al-Drobi, a senior official with the Syrian Brotherhood, said there had been no official change from foreign powers in terms of promises to step up aid to the rebels.
He said the Coalition's election on Saturday, which brought the Saudi-backed tribal leader Ahmed al-Jarba to the helm, should give the West the confidence now to send promised aid.
"We hope the international community will act seriously as it had promised. Up to this point, foreign support has been too minimal to offer any change," Drobi told Reuters by telephone from the conference in Istanbul.
"They have officially welcomed our new leader. Now they need to take the practical steps required."
Washington began to see arming the rebels as more pressing after Assad and Hezbollah forces began operations that seem aimed at securing a belt of territory between the capital and his stronghold and the Syrian coast. That would divide the north and south of Syria, where rebels hold swathes of territory.
The decision to give military support was described as a bid to maintain a "balance of power" that the United States saw as necessary to revive any political negotiations with Assad, who is being backed by Iran and Russia.
Most rebel aid is expected to be sent by the countries' Gulf Arab allies, Saudi Arabia and Qatar. A few shipments of shoulder-launched surface-to-air missiles have been sent to rebels from Saudi Arabia, Gulf officials have said, but rebels say it is still not enough to turn the tide.