Assad has faced international criticism for overseeing a crackdown against demonstrators expressing frustration with his regime.
The Syrian government has rolled out a series of reforms it claims are meant to allay protestors' concerns. Those pledges were met with near-global skepticism as the death toll continues to rise in Syria.
U.N. Secretary-General Ban Ki-moon, in a statement through his office, expressed "deep concern" over the level of violence in Syria, telling authorities there to stop repression "immediately."
"The mass arrests of protesters are not consistent with serious reform and should stop," the statement read. "The secretary-general urges President Assad to concretely respond to pressing grievances and longer term concerns of the Syrian people."
Human Rights Watch said it had evidence to suggest at least 2,000 anti-government protesters and medical workers were arrested by security forces since late June.
The U.N. Security Council has been mum on the unrest in Syria.
At least 16 people were killed Tuesday in the city of Homs, as Syrian dissidents said armed supporters of Assad fired at civilians.