The US has condemned the Syrian army's attack on Qusair, a strategic town over which it gained control after a siege.
The White House also called on Iran and its Lebanese ally Hezbollah to withdraw fighters from Syria, where they have been helping government troops.
A BBC team that visited Qusair found that it was in ruins.
Meanwhile, activists and the Israeli military said a UN-operated border crossing in the Golan Heights had been taken by rebels from Syrian forces.
"The rebels have seized the crossing near the old city of Quneitra in the occupied Golan Heights," Rami Abdelrahman, head of the UK-based Syrian Observatory for Human Rights, told Reuters news agency, adding that fierce fighting was continuing in the area.
The BBC's Yolande Knell, in Jeruslaem, says Israeli officials have voiced fears that the civil war in Syria could spill over their borders and with Islamist extremists among the rebel forces, they're worried that the Golan Heights could be used to launch attacks against Israel in future.
No building escaped damage
On the international front, France said growing proof of chemical weapons use in Syria "obliges the international community to act".
However, President Francois Hollande cautioned: "We can only act within the framework of international law".
He was speaking hours after Syrian government forces backed by Hezbollah fighters retook full control of Qusair, after three weeks of heavy fighting.
A team from the BBC were the first Western journalists to reach the city, and said they did not see a single building that had escaped damage.
More than 80,000 people have been killed in Syria and more than 1.5 million have fled the country since an uprising against President Bashar al-Assad began in 2011, according to UN estimates.
International efforts to resolve the conflict continue, but the US and Russia have failed to set a date for proposed peace talks.
The UN and Arab League envoy to Syria, Lakhdar Brahimi, said the international conference might now be held in July, rather than June as had been planned.
He called the lack of agreement between Washington and Moscow "embarrassing", but also noted that neither side in the Syrian conflict was ready to commit to attending.
Qusair lies only 10km (6 miles) from the Lebanese border and is close to important supply routes for both the government and rebels.
It had been the focus of fighting between rebels and troops backed by a pro-government militia and fighters from Hezbollah, the militant Lebanese Shia Islamist group allied with Iran.
Syrian state TV reported on Wednesday that a large number of rebels had died and many others had surrendered as troops advanced swiftly.
The rebels said they withdrew overnight in the face of a massive assault.
The town where 30,000 people once lived is now all but deserted by civilians, reports the BBC's Lyse Doucet, who was taken to Qusair by the Syrian government.
She found Syrian troops and Hezbollah fighters were everywhere - travelling in trucks and armoured vehicles, firing guns in celebration and moving on foot through the streets.
White House spokesman Jay Carney said the US condemned the assault "in the strongest possible terms".
"It is clear that the regime is unable to contest the opposition's control of a place like Qusair on their own, and that is why they are dependent on Hezbollah and Iran to do their work for them," he said.
The US has called on Hezbollah and Iran to immediately withdraw their fighters from Syria, calling on all parties to allow humanitarian agencies safe access to the area.
Hezbollah - or the Party of God - is a political and military organisation in Lebanon made up mainly of Shia Muslims. It emerged with financial backing from Iran in the early 1980s and has always been a close ally of Syria's.
One of the group's fighters told The Times newspaper it had dispatched some 1,200 special forces fighters to spearhead the assault on Qusair.
"The buildings were so close, we were clearing them not metre by metre but centimetre by centimetre," said the veteran fighter, who went by the nom-de-guerre Haji Abbas and said he had recently returned from a week's fighting in the town.
"We squashed them into the northern part of the town and then pinned them down with sniper fire."
Correspondents say the battle for Qusair has highlighted Hezbollah's growing role in the Syrian conflict - a development that has heightened sectarian tensions in the wider region.
Late on Wednesday Lebanese media said that several rockets had landed in the Hezbollah stronghold of Baalbek inside Lebanon.
While Iran politically and militarily backs the Syrian government, it is not clear that Iranian forces were on the ground during the battle for Qusair.
The International Committee of the Red Cross (ICRC) has said it fears there are shortages of food, water and medical supplies in the town.
George Sabra, the interim chairman of the main opposition alliance, the National Coalition, said there were hundreds of injured people awaiting help in the area and called on the Red Cross to be allowed access.
In Paris, Mr Hollande told reporters: "We have the elements which now allow us to give certainty over the use of chemical weapons in Syria - at what level we still do not know.
"What has happened in Syria must be one more piece of pressure that can be put on the Syrian regime and its allies."
His comments followed those of French Foreign Minister Laurent Fabius, who revealed on Tuesday that samples taken from locations of alleged chemical weapon attacks in Syria, including Saraqeb and Jobar, and brought to France had tested positive for the nerve agent, sarin.
Mr Fabius said he had "no doubt" that sarin had been used by "the Syrian regime and its accomplices", but did not specify instances of its use. The US says more proof is needed.
The Syrian government has repeatedly denied using chemical weapons, and has in turn accused the rebels of doing so, an allegation that they have also rejected.