More than 70 Syrian military officers have defected to the opposition and crossed into Turkey, an official there said on Saturday, as world leaders prepared to discuss the Syrian conflict at the G8 summit.
The defections followed a US decision to give the rebels "military support" after Washington reviewed evidence showing the Syrian regime had used chemical weapons.
But Russia said the regime, which bombarded the outskirts of Damascus on Saturday, had no need to use chemical weapons because its forces were making steady advances on the ground.
The defection of 71 army officers and two policemen came over a period of 36 hours, the Turkish official told AFP.
The group included six generals and 22 colonels, the official added, and was the highest-level defection in months.
Meanwhile, US President Barack Obama held talks on Syria with European leaders late on Friday, the White House said.
The discussions with British Prime Minister David Cameron, French President Francois Hollande, Italian Prime Minister Enrico Letta and German Chancellor Angela Merkel came ahead of the G8 summit to be held in Northern Ireland next week.
"The five leaders discussed Syria, including the regime's use of chemical weapons against its own people, and ways to support a political transition to end the conflict," the White House said on Saturday.
The Syrian conflict is expected to dominate the two days of meetings, with the US pledge to arm the opposition likely to be a topic of discussion.
On Thursday, Washington said it had evidence that the Syrian regime had used chemical weapons, including the nerve gas sarin, in attacks that killed up to 150 people.
It also said without giving details that the administration had decided to boost its support for the rebels seeking to overthrow President Bashar al-Assad, including "military support."
That decision, and the allegations of chemical weapons use by the regime, were criticised by Russia, which is a staunch Assad ally.
Moscow has said it is unconvinced by claims of chemical weapons use in Syria and Foreign Minister Sergei Lavrov said on Saturday that the regime had no need to use such arms.
"The regime, as the opposition is saying out in the open, is enjoying military success on the ground," he told reporters during a joint press appearance with his Italian counterpart Emma Bonino.
"What sense is there for the regime to use chemical arms -- especially in such small amounts?" Lavrov asked.
He warned that it would be wrong for the US administration "to be sending signals" to the opposition that could jeopardise attempts to find a political solution to the Syrian conflict.
Despite supporting opposite sides in the fighting, Moscow and Washington have both backed holding a peace conference in Geneva in coming weeks.
The Syrian government is reportedly willing to attend any such meeting, but the opposition has set conditions for its attendance, including the withdrawal of regime-allied Lebanese Hezbollah from Syria.
On the ground, Syrian regime forces bombed rebel positions around Damascus on Saturday, the Syrian Observatory for Human Rights watchdog said.
The group, which relies on a network of activists, lawyers and doctors, reported air strikes on the eastern neighbourhoods of Jubar and Barzeh.
Syrian state news agency SANA said the army had "restored security and stability" to the village of Ahmadiyeh and parts of Khamissiyeh, in the Eastern Ghouta region of Damascus province.
Eastern Ghouta has been the regular scene of clashes between rebels and government troops in recent months.