"Qusair itself is described as a ghost town, heavily damaged and filled with the sound of bombs. People are hiding in bunkers or, even worse, in holes that they've dug. One woman told us that she spent, with her children, one week inside a hole that was dug into the ground," UNHCR chief spokeswoman Melissa Fleming told a U.N. briefing in Geneva.
Qusair, which lies close to the Lebanese border, was home to about 30,000 people before the war, according to the U.N. children's agency UNICEF. It is now the latest flashpoint of the two-year-old conflict, a strategic prize that both sides need to secure their supply routes.
With the fighting dragging into its third week, Syrian forces fired ground missiles and launched a series of air raids on the besieged town on Tuesday, activists said.
Earlier quick advances made by forces loyal to Syrian President Bashar al-Assad and the allied Hezbollah militia have slowed as they tried to seize the northern quarter of the town. Fighters on both sides say that the advance has slowed to only a few metres in the past few days, but they offer conflicting reasons why.
Rebels say backup opposition forces that broke through the encirclement of Qusair and sent in hundreds of fighters have given rebels a morale boost. They say they have been able to seize some tanks and ammunition and block a few attempts to storm opposition-held areas.
"We will keep fighting, God willing. If you hear the news that Qusair has fallen, you will know that not a single fighter is left in Qusair and that all have been martyred," said one rebel, Abu Tareq, speaking by Skype.
"We will not make a 'strategic' retreat as the opposition has done in previous battles. We will stand our ground to the last man."
Sources close to Assad's forces said the army and Hezbollah fighters had built platforms over the nearby Orontes river to speed up the movement of their troops. They said that the slow advance was planned, and not a loss of momentum. Rebels have mined most buildings and streets in the areas Assad's forces have been trying to storm.
Syrian officials have said they are fighting terrorists in the town and accused U.N. human rights chief Navi Pillay, who said before the battle that local people feared they were about to be killed, of "imagining" a massacre.
One rebel fighter, who asked not to be named, feared a repeat of Baba Amr, a district of the town of Homs that government forces surrounded for months and eventually stormed, forcing the rebels to withdraw.
"If the regime manages to get in, just like in Baba Amr we will see hundreds dead and buildings razed," the fighter said. "Our estimate is that the regime will make an attempt to storm the northern quarter of Qusair around the end of the week."
Some rebels reported seeing gas masks being distributed by Assad's forces, but there was no evidence to back up the claim.
Refugees from Qusair have begun turning up in the Lebanese town of Arsal after a hike of about 100 km, Fleming said. Others have arrived in Jordan, 15 days after leaving Qusair. "They tell us of an extremely difficult journey that they make. Fighters are said to be targeting people as they flee. There are checkpoints all over the place. No route out of Qusair is considered safe," Fleming said.
"We can confirm that most of those who have fled are women and children and this is a bit ominous because they say it is unsafe to flee with men, who have a heightened risk of being arrested or killed at checkpoints along the way."
Others have only got as far as neighbouring towns such as Qara, Nabek and Hasyah, she said, while between 700 and 1,500 injured civilians are also thought to be still in Qusair. Opposition leaders called for the creation of a humanitarian corridor to allow people from Qusair to flee to Lebanon.
"We have in Qusair more than 1,000 wounded, and 400 of them are in critical condition. Some of them have been bleeding for days," said George Sabra, acting head of the opposition National Coalition.
"There are not enough supplies in the city to preserve people's lives. Their lives are in great danger if emergency relief is delayed," he said.