Rebels fought to halt an advance by President Bashar al-Assad's forces into northern Syria on Monday.
New evidence emerged of escalating foreign support for the rebels, with a Gulf source telling Reuters that Saudi Arabia had equipped fighters for the first time with shoulder-fired anti-aircraft missiles, their most urgent request. Rebels said Riyadh had also sent them anti-tank missiles.
European nations backing the rebels would "pay the price" if they joined those sending weapons to Syria, President Bashar al-Assad told a German newspaper.
The Saudi weapons deal was disclosed as rebel fighters confront government troops and hundreds of militants from the Iranian-backed Hezbollah militia seeking to retake the northern city of Aleppo, where heavy fighting resumed on Monday.
After months of indecision, the Obama administration announced last week that it would arm the rebels because Assad's forces had crossed a "red line" by using nerve gas. That has put Washington on the opposite side of the two-year-old civil war from its Cold War foe Moscow, which supplies weapons to Assad.
The United Nations has urged all sides to stop sending arms to a conflict that has killed at least 93 000 and shows no sign of abating. But those calls have been ignored, with regional and global powers doubling down on support for either side.
The White House said last week Obama would try to persuade Putin to drop support for Assad at a summit of the G8 group of world powers in Northern Ireland.
Putin showed no sign of being convinced. Speaking on the summit's eve, he hammered home his point that arming fighters was reckless, zeroing in on an incident last month when a rebel commander was filmed biting a piece of an enemy's entrails.
"One does not really need to support the people who not only kill their enemies, but open up their bodies, eat their intestines in front of the public and cameras," Putin said after meeting British host David Cameron.
Canadian Prime Minister Stephen Harper was equally blunt, saying Putin was supporting thugs.
"We are not – unless there is a big shift in position on his part – going to get a common position with him at the G8."
Obama and Putin are due to meet at about 1730 GMT.
Russia says it is unconvinced by US evidence accusing Assad of using chemical weapons, and said on Monday it would block any attempt to impose a no-fly zone over Syria, a step Washington says it has not yet decided on but is on the table.
The United States moved anti-aircraft missile batteries and warplanes to Jordan in recent weeks, which Moscow believes are a precursor to a no-fly zone.
Assad's forces gain
Those backing the rebels – including Britain, France, Turkey and Arab countries as well as the United States – were driven to intensify support in recent weeks to rescue the rebellion after Assad's forces scored important military gains.
Just a few months ago, Western countries thought Assad's days were numbered. But last month he received the open support of thousands of fighters from Hezbollah, the powerful Iran-backed Shi'ite militia in neighbouring Lebanon, which helped him capture the strategic town of Qusair from the rebels this month.
Hezbollah's participation turns the internal conflict into a regional sectarian battle, with Sunni-ruled Arab states backing the mainly Sunni Syrian rebels and Shi'ite Iran backing Assad, an adherent of the Alawite offshoot of Shi'ite Islam.
The news that rebels are receiving shoulder-fired anti-aircraft missiles from Saudi Arabia marks an important escalation with the potential to shift momentum by limiting Assad's use of air power, one of his major advantages.
The missiles have been at the top of the wish-list of the main rebel military commander, Salim Idriss. The West had long been wary because of fears such weapons could fall into the hands of terrorists and be used to shoot down civilian planes. US ally Israel is particularly fearful of such weapons being distributed in the Middle East.
The missiles were obtained from suppliers in France and Belgium, and France had paid to ship them to the region, the Gulf source told Reuters. France says it has not yet decided to arm the rebels but along with Britain it persuaded the European Union to drop a weapons embargo from the start of this month.
Opposition sources in Aleppo said Saudi Arabia had also supplied the rebels with at least 50 Russian-made Konkurs anti-tank missiles in the last few days. The weapons had reached rebels fighting a government column at the town of Maaret al-Arteek north of Aleppo, scene of major fighting in recent days.
Western countries say helping the rebels on the ground is necessary to restore military balance after Assad's recent gains. Previously, the West provided only "non-lethal" aid.
"We have all understood that you can't win this just with night vision goggles," said one Western official. "You can't engage in a peace process when you come to the table with the balance of power turning against you."
Fighting near Aleppo
After winning the battle for the strategic town of Qusair this month, Assad's forces have announced an offensive in the mainly rebel-held north near Syria's biggest city, Aleppo.
Speaking from Aleppo, a member of the opposition Sham News Network said fighting resumed on Monday around Maarat al Arteek on the northwest outskirts of the city, where Assad's forces had taken high ground threatening rebel positions.
He said 200 Hezbollah fighters had deployed in the two Shi'ite enclaves of Nubbul and Zahra further northwest.
"The fighting is heavy and the regime has the high ground but it has not been able to advance. It aims to control a line from Nubbul and Zahra to Aleppo and build on that to retake the northern countryside," said the activist, who identified himself as Majed. "But the rebels know the terrain and know that enough supplies are coming to achieve victory."
Activists said two people were killed in an air raid on the town of Daret Ezza northwest of Aleppo. Video footage showed people milling around piles of rubble which they identified as remains of a school used to house displaced people.
The Britain-based Syrian Observatory for Human Rights also reported clashes between pro-Assad fighters from Nubbul and Zahra and rebels trying to stop them reaching a government helicopter base at Minagh, which rebels have been trying to capture for a year. – Reuters