Syrian rebels braced on Saturday for a new assault on their beleaguered stronghold of Qusayr by the army and its Hezbollah allies, who were bolstering positions north of the town, a watchdog said.
Fears of another move on the town renewed concerns about the fate of an unknown number of civilians still trapped there, including an estimated 1,000 wounded.
UN Secretary General Ban Ki-moon demanded that both sides allow civilians to flee.
Rami Abdel Rahman, director of the Syrian Observatory for Human Rights watchdog, told AFP that "there are ongoing clashes in northern Qusayr, and 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 added.
The group said at least 15 tanks were massed north of Qusayr, a key strategic prize for both the regime and the rebels.
It sits on the route linking Damascus to the Mediterranean coast, and lies near the Lebanese border, providing a key rebel conduit for weapons and fighters.
The Syrian opposition said on Friday that rebel reinforcements had reached the area.
The fight for the town, which began nearly two weeks ago, has raised fears about the safety of thousands of civilians.
Abdel Rahman, whose watchdog relies of a network of activists, doctors and lawyers on the ground, said around 1,000 wounded people were trapped inside Qusayr.
"The medical situation is very bad," he said.
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.
"He also reminds the government of its responsibility to protect civilians who come under its control, including from the threat of militias. He calls on the warring parties to allow trapped civilians to escape the town."
The opposition Syrian National Coalition saluted rebels in Qusayr, including the reinforcements.
"The heroes of the Free Syrian Army prove every day that they are worthy of the responsibility that the people have entrusted them with," it said.
"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."
The powerful Lebanese Shiite group, a staunch ally of President Bashar al-Assad's regime, has dispatched 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 throughout the region to follow suit and join the Syria uprising.
"Every Muslim trained to fight and capable of doing that (must) make himself available" to support the Syrian rebels, the cleric said at a rally.
"Iran is pushing forward arms and men (to back the Syrian regime), so why do we stand idle?" added Qaradawi, a controversial figure who has millions of supporters, mostly from the Muslim Brotherhood.
Despite an official policy of neutrality on the Syria 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 injuries.
And Lebanon's National News Agency reported that unidentified gunmen opened fire overnight at a Shiite shrine in Baalbek, a Hezbollah stronghold also in the Bekaa.
The continued fighting has raised concerns about the prospects for a peace conference expected to convene in Geneva this month to seek a political solution to the conflict.
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".
And Ban warned that, as preparations for the conference intensify, all parties to the conflict "will be held accountable for any acts of atrocity carried out against the civilian population of Qusayr".
In other developments, the Observatory said an Islamic court in the northern city of Aleppo had executed the commander of the Mohammed's Army Brigade and one of his aides on charges of murder, theft and bribing local residents.
The group also reported the execution of a man accused of helping government forces, saying his body was hanged in a local square in Damascus province.
In the capital itself, the Barzeh district came under heavy shelling as regime forces battled rebels.
The Observatory said at least 55 people had been killed throughout the country on Saturday -- 27 civilians and 28 rebels.