(CNN) -- The death toll in Syria's violent crackdown on anti-regime protesters has risen to close to 5,000 people, the United Nations high commissioner for human rights said Monday.
"This situation is intolerable," Navi Pillay said in a briefing for the U.N. Security Council.
More than 300 hundred of dead have been children "killed by state forces," Pillay said, citing "reliable sources."
Over 200 people have reportedly died in violence in Syria since in the last 10 days, and "the Syrian population continues to live in fear of further violent repression," she said.
"The nature and scale of abuses" indicate that Syrian forces likely committed "crimes against humanity," Pillay said. "Independent, credible and corroborated accounts demonstrate that these abuses have taken place as part of a widespread and systematic attack on civilians."
The Syrian government must have authorized them or at least knew about them, she said. "Several defectors from military and security forces have said that they received orders to shoot unarmed protesters without warning."
Pillay named the city of Homs, one of the uprising's main centers, as a particular flash point.
As nightfall arrived Monday in Syria, many residents of Homs went to bed afraid that a historic siege on their city could be imminent.
Opposition figures said the Syrian government had warned people in Homs to stop anti-government protests, hand in weapons and surrender defecting military members by Monday night -- or face attack by the government forces.
Syrian forces gave a 72-hour warning, said Lt. Col. Mohamed Hamdo of the Free Syrian Army, an opposition group of defected Syrian military personnel. Activists on the ground said the ultimatum was issued Friday for Homs.
Hamdo said Syrians are worried about a repeat of what happened in 1982 when Syria's military -- acting under orders from then-President Hafez al-Assad, father of current Syrian leader Bashar al-Assad -- launched an assault on the city of Hama, killing thousands. "We fear that a similar massacre or worse could take place in Homs," he said.
"People are very afraid," said Wissam Tarif, a human rights activist in Beirut, Lebanon, with the organization Avaaz, who is in touch with people in Syria.
There are enough troops around Homs "to take over the city," he said, and casualties have been increasing "in very big numbers" over the past couple of days. "People are afraid that the army might now invade the city."
Hamdo said the military has dug trenches around Homs and largely cut it off.
"The situation in Homs is really bad. There is no electricity, water, and the communication lines are much worse. The food supply is also decreasing, mainly because little food is going in," he said.
The Syrian National Council, the country's leading opposition movement, earlier warned of a potential bloodbath in Homs at the hands of the Syrian regime.
The Syrian government denied reports of water and electricity being out in the city, according to the state-run Syrian Arab News Agency.
The government has not acknowledged a deadline for Homs on state-run media.
The Local Coordination Committees of Syria, a network of opposition activists in the country, said Monday the Syrian army and security forces killed 21 people, including four women and three children. Thirteen of the deaths were in Homs, three were in Hama, three in Damascus suburbs and two in Idlib.
Meanwhile, state TV painted a picture of normalcy, with reports of local elections under way across the country Monday.
The state-run Syrian Arab News Agency (SANA) noted that more than 3,000 candidates are vying for seats in the Homs region alone. It billed the elections as part of the "process of building institutions, promoting democracy and achieving the comprehensive reform process led by President Bashar al-Assad."
Despite that picture, fierce clashes broke out between security forces and defectors in the cities of Daraa and Idlib, according to the London-based Syrian Observatory for Human Rights. The organization collects information from people in different parts of the country.
Hamdo of the Free Syrian Army said, "We conducted an operation late last night against the Syrian forces in Idlib and killed eight of them and injured 22. Two of our men are critically injured. "
Syrian forces were conducting mass arrests of shop owners who shut their stores Sunday as part of a nationwide anti-government strike, Hamdo said.
Via SANA, the Syrian government on Sunday quoted people saying there was no strike and no sign of a strike.
Throughout the uprising, Syria has insisted it is not targeting peaceful protesters and instead cracking down on armed terrorist gangs.
Reports of deaths in clashes between demonstrators and government forces have escalated overthe last nine months as protesters demand democratic elections and the end of al-Assad's regime. Al-Assad has been in power since 2000; his father, Hafez, ruled Syria for three decades.
CNN cannot independently confirm events because the Syrian government restricts access of international media to the country.
World leaders have widely condemned Syria's crackdown and called on it to halt violence against the opposition.
The Arab League announced it will hold emergency meetings this week in Cairo. In a statement on Egypt's state-run MENA news agency, an Arab League official said leaders will "discuss the Arab response to a message from Syrian Foreign Minister Walid Moallem to approve the signing of an agreement on an Arab League observing mission to Syria with conditions."
On Saturday, France expressed its concerns, warning Syria about launching a military operation against Homs and its population.
Israeli President Shimon Peres on Sunday described Syria's president as a "killer," implicitly comparing him to Libya's Moammar Gadhafi.
"The world decided -- including the Arab world -- to intervene when a leader is beginning to kill his own citizens," Peres told CNN. "It happened in Libya; it's happening in Yemen; it's happening by the Arab League, for the first time in their experience. They decided to put pressure on an Arab state because the leader is killing his people."
CNN's Hamdi Alkhshali, Amir Ahmed, Josh Levs and Richard Quest and journalist Mohamed Fadel Fahmy contributed to this report.