(CNN) -- More than 2,200 people have been killed since the start of mass protests in Syria in mid-March, the U.N. High Commissioner for Human Rights said on Monday.
Navi Pillay spoke in Geneva, Switzerland at a special session of the U.N. Human Rights Council, which met to discuss allegations of human rights violations in Syria.
The meeting comes after the United Nations sent a humanitarian mission to Syria this weekend to look for effects of President Bashar al-Assad's protest crackdown.
Pillay said that more than 2,200 people have been killed in the crackdown so far, with more than 350 people reportedly killed since the start of Ramadan.
"I wish to use this opportunity to once again call on the Syrian government to immediately and fully halt its crackdown on peaceful protests and ensure the immediate and unconditional release of all detained for their participation in peaceful demonstrations," she said during her remarks, according to a U.N.-provided transcript.
Meanwhile in Syria, the Syrian Observatory for Human Rights reported new deaths in a continuing crackdown on pro-reform demonstrators.
Two people were killed and four wounded early Monday morning in the city of Mesyaf in Hama province when regime supporters opened fire, the London-based activist group said.
It also said that at least three people were killed and five injured in Homs, when gunmen opened fire on demonstrators participating in a protest timed to coincide with the U.N. delegation's visit.
On Sunday, al-Assad rejected calls for his ouster and promised steps toward political reform.
Speaking on Syrian state TV a day after the U.N. mission arrived in the country, al-Assad was asked about calls from European leaders for him to step down. "What they say means nothing to us," he replied, according to a CNN translation.
U.S. President Barack Obama made the same call last week, and the United States followed up by imposing new economic sanctions.
The European Union's political security committee is considering an embargo on Syrian crude oil. Oil and gas make up about a quarter of Syria's economy, according to the International Monetary Fund.
In his televised interview, al-Assad continued to deny that his military has targeted peaceful protesters, despite widespread reports from witnesses, medical officials and diplomats in the country.
The U.N. team's arrival in Syria came just days after a U.N. fact-finding mission reported having found "a pattern of human rights violations that constitutes widespread or systematic attacks against the civilian population, which may amount to crimes against humanity."
The fact-finding mission's report called on the Security Council to "address in the strongest terms the killing of peaceful protesters and other civilians in Syria through the use of excessive force and other grave human rights violations; to call for an immediate cessation of attacks against the civilian population; and to consider referring the situation in Syria to the International Criminal Court."
CNN's Arwa Damon, Kamal Ghattas, Josh Levs and Chelsea J. Carter contributed to this report.