Aid organisations stepped up calls for civilians trapped in the flashpoint Syrian town of Qusayr to be evacuated, as rebel fighters braced for a fresh assault from regime forces.
UN agencies and the International Red Cross appealed to both sides in the fighting to let the civilians, including 1,500 wounded according to UN figures, leave the embattled town.
Saturday saw fierce fighting in the north of the town between rebel fighters and government troops backed by its Hezbollah allies, said a watchdog.
UN Secretary General Ban Ki-moon called on both sides to allow civilians to leave, and the organisation's rights chief and relief coordinator called for a ceasefire so they could do so.
Rami Abdel Rahman, director of the Syrian Observatory for Human Rights watchdog, told AFP on Saturday that "the opposition fighters are fighting with everything they've got.
"Regime forces are reinforcing the sites that they have north of the city, including Dabaa airport and Jawadiya," he said.
The group said at least 15 tanks were massed north of Qusayr, a prize for both the regime and the rebels as it links Damascus to the coast, and is near the Lebanese border, providing a key rebel conduit for weapons and fighters.
The town has been the scene of fierce fighting for two weeks now.
With fears growing for the civilians there, Abdel Rahman said around 1,000 wounded people were trapped inside Qusayr. UN officials suggested the figure could be even higher.
"We understand there may also be as many as 1,500 wounded people in urgent need of immediate evacuation for emergency medical treatment, and that the general situation in Al-Qusayr is desperate," said a joint statement by UN humanitarian chief Valerie Amos and UN human rights commissioner Navi Pillay.
They were "extremely alarmed" by reports that civilian neighbourhoods were under fire, they added.
"We urge the parties to agree to an immediate ceasefire to allow humanitarian agencies to evacuate the wounded and provide life-saving treatment and supplies."
In New York, UN chief Ban Ki-moon urged "all sides to do their utmost to avoid civilian casualties," spokesman Martin Nesirky said in a statement.
Ban also called on all sides to help trapped civilians escape the town.
In Geneva, the International Committee of the Red Cross (ICRC) also expressed alarm.
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"Civilians and the wounded are at risk of paying an even heavier price as the fighting continues," said Robert Mardini, head of Middle East operations.
He called for restraint on all side, adding that the ICRC had already requested access to the town.
The opposition Syrian National Coalition said on Friday that rebel reinforcements had reached the town and paid tribute to the fighters.
"The people will continue their struggle to liberate their land, whatever it takes, and will force Hezbollah to withdraw its forces from all of Syria," it said.
The powerful Lebanese Shiite group, a staunch ally of President Bashar al-Assad's regime, has sent thousands of fighters to help put down the uprising that began more than two years ago with peaceful protests.
Some members of Lebanon's Sunni Muslim community have also crossed the border to fight alongside the Sunni-led rebels, encouraged by local clerics.
Late on Friday, influential Muslim cleric Yusuf al-Qaradawi urged Sunnis to follow suit and join the uprising.
"Iran is pushing forward arms and men (to back the Syrian regime), so why do we stand idle?" he said at a rally.
Qaradawi, a controversial figure, has millions of supporters, mostly from the Muslim Brotherhood.
Despite an official policy of neutrality on the conflict, Lebanon has found itself increasingly embroiled in its neighbour's civil war.
On Saturday morning, at least six rockets fired from Syria struck the Bekaa region, causing no casualties.
And Lebanon's National News Agency reported that unidentified gunmen opened fire overnight Friday at a Shiite shrine in Baalbek, a Hezbollah stronghold in the Bekaa.
The fighting in Syria has raised concerns about the prospects for a peace conference expected in Geneva this month.
The Coalition reiterated on Saturday that "the immediate halt of military operations by regime forces, Hezbollah and Iran are the primary conditions for participation in the conference".
Ban warned that, as preparations for the conference intensified, all parties to the conflict "will be held accountable for any acts of atrocity carried out against the civilian population of Qusayr".
The Observatory said at least 69 people were killed nationwide on Saturday, mostly in Homs and Damascus provinces.