Syrian government forces have taken full control of the strategic town of Qusair, state TV and the rebels say.
The town, near the Lebanese border, has been the focus of fierce fighting for more than two weeks between rebels and Syrian troops backed by fighters from the Lebanese militant group Hezbollah.
State TV said 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.
Television pictures showed widespread destruction, with many buildings reduced to rubble and no civilians in sight.
The International Committee of the Red Cross (ICRC) said it hoped to gain access to the town to help civilians caught up in the fighting.
"There are reports of hundreds of people that have been wounded that have not been able to receive the necessary medical treatment," ICRC spokeswoman Rima Kamal told the BBC.
She said there were also reports of shortages of food, water and medical supplies.
George Sabra, the chairman of the Syrian National Coalition, the main umbrella opposition group outside Syria, said that there were hundreds of injured people awaiting help in the area and called on the Red Cross to be allowed access.
Qusair's fall comes as plans have stalled for an international peace conference to find a political solution to the two-year-old Syrian conflict.
The UN and Arab League envoy to Syria, Lakhdar Brahimi, said the talks might now be held in July, rather than June as had been planned. He called the lack of agreement on a date among the US and Russia "embarrassing".
Also on Wednesday, French President Francois Hollande said mounting proof of chemical weapons use in Syria "obliged the international community to act", adding that any action had to be "within the framework of international law".
The Syrian government hailed the capture of Qusair - which had been in rebel hands for more than a year - as an important strategic victory.
The town lies just 10km (six miles) from the Lebanese border and is close to important supply routes for both sides in the conflict.
The army said Qusair's capture was "a clear message to all those who share in the aggression on Syria... that we will continue our string of victories until we regain every inch of Syrian land".
The state news agency, Sana, reported that a large number of "terrorists", as the government refers to the rebels, had been killed and many had surrendered.
It said the army was now destroying barricades and weapons caches, and searching the town for booby-traps.
One reason given by the rebels for their decision to pull out overnight was that they have been starved of weapons, despite the EU's decision to lift the embargo on supplies to them, the BBC's Jim Muir reports from Beirut.
"In the face of this huge arsenal and lack supplies and the blatant intervention of Hezbollah... tens of fighters stayed behind and ensured the withdrawal of their comrades along with the civilians," said a rebel statement quoted by Reuters.
The military leader of the main rebel umbrella group, the Free Syrian Army, had earlier said his fighters were prepared to take the conflict inside Lebanon in pursuit of Hezbollah fighters.
The battle for Qusair has highlighted the growing role of the Lebanese Shia militant group in the Syrian conflict - a development that has heightened sectarian tensions in the wider region.
The fall of the town has symbolic importance in the run-up to a proposed peace conference, because neither side wants to go into the talks looking weak, our correspondent says.
Russian and US officials met in Geneva on Wednesday to try to arrange a date and other details of the conference, but were only able to agree on another date for more preparatory talks at the end of June.
Hundreds of Syrian refugees have been pouring across the border to the Lebanese town of Arsal, our correspondent reports, many of them attempting to register with the local municipality for supplies of water, food and other supplies.
More than 80,000 people have been killed in Syria and more than 1.5 million have fled the country since an uprising against Mr Assad began in 2011, according to UN estimates.
France's Foreign Minister, Laurent Fabius, said on Tuesday that samples taken from Syria and tested in France showed the nerve agent sarin had been used in a localised manner.
The Syrian government has repeatedly denied using chemical weapons, and has in turn accused the rebels of doing so, which they have also rejected.