The military chief of the main umbrella group of Syrian rebels says his men are ready to fight Hezbollah militants inside neighbouring Lebanon.
Gen Selim Idriss of the Free Syrian Army said that Hezbollah fighters were "invading" Syria and Lebanon was doing nothing to stop them.
He denied rebels were losing the war, despite recent setbacks.
Gen Idriss also said the rebels would not accept any role for President Bashar al-Assad in a post-conflict Syria.
"If the price of peace is that Bashar still has the power in Syria, we don't need that kind of peace," he said.
He said he had told commanders, including those near the strategic town of Qusair, to fight Hezbollah militants "just inside Syria".
Syrian forces retake Qusair
Syrian pro-government forces have taken full control of the strategic town of Qusair, state TV and the rebels say.
The BBC reports that the town, near the Lebanese border, has been the centre of fighting for more than two weeks between rebels and Syrian troops backed by fighters from the Lebanese militant group Hezbollah.
Syrian state TV said a large number of rebels had died and many had surrendered.
The rebels said they withdrew overnight in the face of a massive assault.
France says tests confirm sarin used in Syria
The French government says tests show the nerve gas sarin has been used several times during the two-year civil war in Syria.
France's foreign minister Laurent Fabius said laboratory tests in Paris confirmed usage of the nerve agent, but did not specify where, when or by whom it was deployed.
The White House said more proof was needed, the BBC reports.
Earlier, a United Nations report said there were reasonable grounds for believing chemical weapons had been used by both sides.
In its report, the UN commission of inquiry on Syria said the country's civil war has reached new levels of brutality and war crimes and crimes against humanity have become a daily reality.
The commission has so far been barred from Syria and was forced to rely on first-hand accounts from the country.
The report accuses both sides of abuses, but says rebel actions have not reached the intensity and scale of abuses committed by government-allied forces.
It said children have been taken hostage, forced to watch torture and even take part in beheadings, and others have been killed while fighting.