The United States said Sunday it would double its aid to Syria's opposition, including with new non-lethal military equipment, but paid no heed to calls for arms supplies or a direct intervention.

In a statement after talks among the pro-opposition "Friends of Syria" group in Istanbul, Secretary of State John Kerry said US assistance to the opposition would double to $250 million (191 million euros).

Some of the money will be used to "provide an expanded range of support" to rebel fighters battling President Bashar al-Assad, beyond the current provisions of food rations and medical kits, "to include other types of non-lethal supplies," the statement said.

It did not elaborate, but US media had reported that Washington is preparing to provide the rebels with protective battlefield equipment such as body armour, armoured vehicles and night-vision goggles, as well as communications gear.

"The president directed me to step up our efforts," Kerry told a news conference.

"The stakes in Syria couldn't be more clear: chemical weapons, the slaughter of people by ballistic missiles and other weapons of huge destruction, the potential of a whole country," he said.

"This bloodshed needs to stop."

But the pledge fell short of opposition demands for foreign backers to supply the rebels with arms, institute a no-fly zone and carry out airstrikes on positions used by the Assad regime to launch missiles.

Many in the West have raised concerns about arming the rebels, fearing weapons could end up in the hands of radical Islamist groups such as the Al-Nusra Front, which this month pledged allegiance to Al-Qaeda.

Western diplomats have said increased military support will hinge on the opposition showing it can be more inclusive and that it could ensure weapons would be secure. They would also have to reject the use of chemical weapons and guarantee respect for human rights.

The head of the main opposition Syrian National Coalition, Ahmed Moaz al-Khatib, offered reassurances after the talks.

"Our revolution is for the entire Syrian people. We are not supporting one group at the price of another and we shall never allow that to happen," he told the news conference.

The Coalition said in a separate statement that the opposition rejected "all forms of terrorism and any extremist ideology" and promised that "weapons will not fall in the wrong hands".

The opposition had also voiced frustration earlier at the lack of a strong international response.

"Assad is firing missiles against densely populated areas... without consequences," said Yaser Tabbara, a spokesman for the opposition's interim prime minister Ghassan Hitto.

"Throwing money at the problem won't solve it."

The opposition has called for "surgical strikes" on regime missile batteries used against civilians.

Top diplomats from the 11-nation core group of the "Friends of Syria" -- including the United States, European nations and Arab countries -- took part in the more than six hours of talks on Saturday.

In a joint statement afterwards, they warned Assad that foreign support for the opposition would grow if he continued to rebuff efforts to find a political solution to Syria's crisis.

"If the Syrian regime rejects this opportunity, further announcements regarding expanding our assistance will follow," they said.

All members had committed to providing military aid only through the main opposition military leadership, they added.

The Friends of Syria group has struggled to come up with a united strategy to end the violence in Syria, despite fighting that has seen more than 70,000 killed and hundreds of thousands forced from their homes.

Washington has been gradually shifting its policy on providing assistance to the opposition, with Kerry announcing at the last Friends of Syria meeting in February that the US would start providing direct non-lethal aid to rebel fighters.

Britain and France had been pushing for a European Union arms embargo to be allowed to expire by the end of May. But Paris has appeared more wary about arms supplies since the Al-Qaeda pledge from Al-Nusra.

Supporters of arms supplies have said the rise of Islamist groups such as Al-Nusra is only a stronger argument for providing weapons to more moderate voices in the Syrian opposition.