* Saudi steps up weapons supplies to rebels - Gulf source
* West wants all aid to go through rebel commander Idriss
* Rebels announce offensive in Aleppo; fighting in Damascus (Adds Kerry, diplomatic sources, Aleppo fighting)
International opponents of Syrian President Bashar al-Assad agreed on Saturday to give urgent military support to Western-backed rebels, aiming to stem a counter-offensive by Assad's forces and offset the growing power of jihadist fighters.
Assad's recapture of the strategic border town of Qusair, spearheaded by Lebanese Hezbollah guerrillas, and an expected assault on the divided northern city of Aleppo have alarmed supporters of the Syrian opposition.
The U.S. administration has responded by saying, for the first time, it would arm rebels, while Gulf sources say Saudi Arabia has accelerated the delivery of advanced weapons to the rebels over the last week.
Ministers from the 11 core members of the Friends of Syria group, agreed "to provide urgently all the necessary materiel and equipment to the opposition on the ground," according to a statement released at the end of their meeting in Qatar.
The statement did not commit all the countries to send weapons, but said each country could provide assistance "in its own way, in order to enable (the rebels) to counter brutal attacks by the regime and its allies".
The aid should be channelled through the Western-backed Supreme Military Council, a move that Washington and its European allies hope will prevent weapons falling into the hands of Islamist radicals including the al Qaeda-linked Nusra Front.
Ministers from the Friends group - which includes Western and Arab states as well as Turkey - also condemned "the intervention of Hezbollah militias and fighters from Iran and Iraq", demanding they withdraw immediately.
As well as fighting in Qusair, Hezbollah is deployed alongside Iraqi gunmen around the Shi'ite shrine of Sayyida Zainab, south of Damascus, while Iranian military commanders are believed to be advising Assad's officers on counter-insurgency.
SAUDI SPEEDS UP SUPPORT
Two Gulf sources told Reuters that Saudi Arabia, which started supplying anti-aircraft missiles to the rebels on a small scale two months ago, had accelerated delivery of sophisticated weaponry.
"In the past week there have been more arrivals of these advanced weapons. They are getting them more frequently," one source said, without giving details. Another Gulf source described them as "potentially balance-tipping" supplies.
French military advisers are already training the rebels to use some of the new equipment in Turkey and Jordan, sources familiar with the training programmes said. U.S. forces have been carrying out similar training, rebels say.
Rebel fighters say they need anti-aircraft and anti-tank weapons to stem the fightback by Assad's forces in a civil war that has killed 93,000 people, driven 1.6 million refugees abroad and cost tens of billions of dollars in destruction of property, businesses and infrastructure.
Louay Meqdad, spokesman for the Supreme Military Council led by former Syrian army general Salim Idriss, said it had received several batches of weapons.
"They are the first consignments from one of the countries that support the Syrian people and there are clear promises from Arab and foreign countries that there will be more during the coming days," he told Reuters Television in Istanbul.
A French diplomatic source said Paris would increase non-lethal aid such as communications equipment, gas masks, night vision goggles and bullet proof vests. It would also provide assistance with military strategy and battlefield intelligence.
"All this has already started," a Western source said. "Broadly speaking, Western nations will do this, while Gulf Arab nations will deliver the weapons. It's a division of roles.
"If the northern front receives enough material and non-material support quickly, it could soon be equivalent to thousands of men, or even tens of thousands," the source added.
Idriss himself told Al-Jazeera International television on Saturday that his men were still lacking "effective air defence" against Assad's planes and helicopters.
"That's why we are asking for shoulder-launched anti-aircraft missiles ... and anti-tank missiles, modern ones with long range," he said. "We need it yesterday ... because the regime is trying to recapture the whole country."
The increasingly sectarian dynamic of the war pits mainly Sunni Muslim rebels against forces loyal to Assad - who is from the Alawite minority, an offshoot of Shi'ite Islam - and has split the Middle East along Sunni-Shi'ite lines.
U.S. Secretary of State John Kerry said Hezbollah's role transformed the conflict "into a much more volatile, potentially explosive situation that could involve the entire region".
"Hezbollah is a proxy for Iran, supported by Iran, and obviously Hezbollah has decided to become involved in this in a very, very significant way," he said, declining to give details about what weapons Washington might provide to Syria.
Tehran, which says it supports Assad economically and politically, criticised the Doha participants.
"If a meeting leads to weapons being put in the hands of mercenaries and terrorists in Syria, prolonging the killing in Syria and bringing destruction, we are against it," ISNA news agency quoted Foreign Minister Ali Akbar Salehi as saying.
Prime Minister Sheikh Hamad bin Jassim al-Thani of Qatar, which along with Saudi Arabia has been one of the most open Arab backers of the anti-Assad rebels, said that supplying them with weapons was the only way to resolve the conflict.
"Force is necessary to achieve justice. And the provision of weapons is the only way to achieve peace in Syria's case," Sheikh Hamad told ministers at the start of the talks.
The meeting in Qatar brought together ministers and senior officials of countries that support the rebels - France, Germany, Egypt, Italy, Jordan, Qatar, Saudi Arabia, Turkey, United Arab Emirates, Britain and the United States - although the fractured Syrian opposition itself was notably absent.
Sheikh Hamad said all but two countries had agreed on the kind of support to provide to the rebels. He did not name the dissenters, but Germany and Italy have both said in the past they oppose arming the rebel brigades.
The United States and Russia, which back opposing sides in the conflict, hope to bring them together for negotiations originally scheduled to be held in Geneva this month but now unlikely to take place before August.
In northern Syria, rebels announced an offensive on Saturday that they said aimed to capture the western districts of the city of Aleppo from government forces.
The Syrian Observatory for Human Rights also reported heavy clashes near the town of Safirah, south-east of Aleppo, as Assad's forces sought to secure a supply line north towards the city from the central province of Hama.
In Damascus, the army sustained a bombardment of the eastern rebel-held district of Qaboun and soldiers clashed with rebels in the Barzeh district, the Britain-based Observatory said.