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.
The Syrian opposition yesterday said that rebel reinforcements had reached the area.
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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.
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 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.
Members of Lebanon's Sunni Muslim community have also crossed the border to fight alongside the Sunni-led rebels, encouraged by local clerics.
Late yesterday, influential Muslim cleric Yusuf al-Qaradawi urged Sunnis throughout the region to follow suit and join the Syria uprising.
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.