PARIS (AFP) - US Secretary of State John Kerry hinted at greater US support for Syria's opposition on Wednesday, saying it needs "more help" in the struggle against Bashar al-Assad and that Washington wants to speed up a political transition.

In Paris on the eve of a meeting in Rome of the Friends of Syria group, Mr Kerry said boosting support for the opposition would be a key part of the talks Thursday bringing together foreign powers and the main opposition National Coalition.

"We are examining and developing ways to accelerate the political transition that the Syrian people seek and deserve, and that is what we will be discussing in Rome," Mr Kerry said at a joint press conference with French counterpart Laurent Fabius.

He said he wanted to hear from the opposition about how best to end the violence in Syria, where the United Nations says at least 70,000 have died and hundreds of thousands have been uprooted in the two-year conflict.

"That may require us to change president Assad's current calculation. He needs to know he can't shoot his way out of this," Mr Kerry said. "I think the opposition needs more help in order to be able to do that and we are working together to have a united position." Mr Kerry said there was a desire to help the opposition deliver assistance and basic services in areas it has "liberated from the regime".

The Washington Post reported on Wednesday that the White House was considering a policy shift to supply rebels with "non-lethal" aid, including armoured vehicles and perhaps even military training.

A US State Department official refused to confirm the report, but said Washington wanted to help the opposition maintain "the institutions of the state" in areas under their control.

"We're talking about basic services, water, electricity - but also (to) build up new institutions in terms of governance, rule of law, police," State Department deputy acting spokesman Patrick Ventrell told reporters.

Mr Kerry's remarks came ahead of a weekend opposition gathering in Istanbul to elect a prime minister and government to run "liberated" parts of Syria.

Turkish Prime Minister Recep Tayyip Erdogan on Wednesday called for more support from the international community for Syria's opposition, saying the lack of a clear leader among the opposition was no reason to back Assad's "cruel" regime.

"The struggle of the opposition is important and should be appreciated.

Their effort is the way to prepare the ground for a democratic process to take hold for the Syrian people," Mr Erdogan said at a United Nations event in Vienna.

Also on Wednesday, UN aid agencies said they were struggling to keep up with the worsening conflict as the number of refugees in countries around Syria nears the one-million mark and aid pledges are falling short.

"This is a crisis that is completely stretching our capacity," UN humanitarian chief Valerie Amos told reporters after briefing the UN Security Council, adding that just US$200 million (S$247.4 million) out of US$1.5 billion pledged at a recent aid donors conference had been received.

But combatants in Syria seemed deaf to the diplomacy, with fighting and bombardment reported near Damascus as the regime renewed its campaign to suppress the insurgency.

Mr Kerry had on Tuesday met Russian Foreign Minister Sergei Lavrov for talks in Berlin and while initially at odds over the two-year-old conflict, Washington and Moscow have sought to find common ground.

Russia, the most powerful supporter of Mr Assad, this week urged his regime to open talks to end the conflict, with Mr Lavrov also urging the opposition to "declare itself in favour of dialogue".

Washington has recently toned down its criticism of Moscow's perceived intransigence despite Russia having vetoed UN Security Council resolutions which threatened sanctions against Damascus.

The umbrella opposition National Coalition cancelled a planned boycott of the 11-nation meeting in Rome after the US and Britain "promised specific aid to alleviate the suffering of our people".

Meanwhile, Damascus has decided to renew the passports of any Syrians abroad, in an apparent concession after Coalition chief Ahmed Moaz al-Khatib demanded such a move as a condition for talks.

Washington is expected to use the Rome meeting to boost the morale of the opposition, which has grown frustrated at the lack of progress on the diplomatic front.

In Rome, "the Americans want to boost the opposition's morale... because they are aware negotiations with the Russians could last several months," said Karim Bitar of the French Institute of International and Strategic Studies.

On Saturday in Istanbul the Coalition is to appoint the head of an interim cabinet in a secret ballot.

Among those tipped to fill the post are Burhan Ghalioun, the former head of opposition faction the Syrian National Council and ex-prime minister Riad Hijab who defected in mid-2012.

On the ground, fierce battles rocked towns near Damascus as the regime renewed its campaign to crush the insurgency around the capital, a watchdog said.

Tanks pounded the rebel-held town of Daraya, southwest of Damascus, while new clashes broke out in Irbin to the north-east, said the Syrian Observatory for Human Rights.