Opposition activists reported a fierce military offensive in the Syrian city of Idlib on Monday, the same day U.N. observers began monitoring a tenuous cease-fire.
Mortar shells and helicopter gunfire pounded the city, according to opposition activists in the neighboring town of Binnish.
Rebel fighters for the Free Syrian Army described the besieged city to activists in Binnish, saying Idlib residents were trapped in their homes as the sound of sniper fire and explosions rang out in abandoned streets.
One activist said the opposition fighters described bodies in the streets but said it was impossible to confirm the death toll amid ongoing violence. Another estimated that at least 100 people had been killed.
The state-run Syrian Arab News Agency quoted an unnamed military source Monday as saying that "armed terrorists" were behind the violence, a claim made repeatedly over the past year by the Syrian regime.
The news agency reported that aggression by the groups had "hysterically escalated" since the start of the cease-fire.
CNN cannot independently verify reports of violence and deaths, as the government has severely restricted access by international media.
The reported violence came as the first members of a U.N. observer team began a monitoring mission in Syria.
The observers will be "liaising with the Syrian government, security forces and the opposition members to establish the monitoring process across the country," said Kieran Dwyer, a spokesman for peacekeeping missions at the United Nations.
The initial group of six observers arrived Sunday, a day after the U.N. Security Council voted unanimously to authorize unarmed observers to travel to Syria to monitor the cease-fire, part of a peace plan brokered by U.N.-Arab League special envoy Kofi Annan.
Syria pledged last week to abide by the terms of Annan's plan, which called for a cease-fire to begin by Thursday morning. Since then, however, opposition groups have reported ongoing violence and killings at the hands of the regime.
At least 27 people were killed across Syria by midday Monday, according to the Local Coordination Committees of Syria, a London-based network of opposition activists.
Seven of those killed were in Idlib, where Syrian army tanks shelled the city and regime forces fired heavy machine guns, the Local Coordination Committees said.
Sniper fire outside Damascus killed a 6-year-old, and the regime executed two soldiers as they were trying to defect in Idlib, according to the opposition Syrian Network for Human rights.
Meanwhile, a U.N. human rights panel tasked with investigating the situation in Syria said Monday that it is "seriously concerned" about accounts of government forces shelling neighborhoods and using heavy weaponry since Thursday.
The Commission of Inquiry on Syria also said it had received "reports of human rights abuses committed by anti-government armed groups engaged in fighting against the Syrian army during and after the cease-fire, including extra-judicial killings of soldiers captured during armed confrontations."
Reported death tolls since Thursday, however, have been lower than those before the deadline. Since April 1, opposition groups had reported more than 50 deaths a day; on four days, they said the death toll was more than 100.
By contrast, the Local Coordination Committees reported at least 37 deaths Thursday, at least 13 on Friday, 30 on Saturday and 28 on Sunday.
Armed opposition fighters said Monday that they aren't waiting to see how the cease-fire holds. They are gathering more weapons to fight the regime just in case the agreement falls apart.
Capt. Amar Wawi, leader of the Ababil Battalion of opposition fighters based in Aleppo, said the rebel Free Syrian Army has modern equipment including some anti-tank missiles -- but he declined to give details.
"We are preparing ourselves for the next stage if the Annan mission fails," Wawi said from the Syria-Turkey border. "We will then use this equipment against the Assad thugs."
The Free Syrian Army is a network mostly of Syrian military defectors. It has no leadership structure, however, with members fighting in separate groups or battalions operating in different towns.
Lt. Abdullah Oda, an opposition fighter in Istanbul, said he was in Iraq last week brokering a deal to send weapons, including anti-tank missiles, to the Free Syrian Army.
"They got this equipment from rebel supporters in the Iraqi-Syrian border," Oda said. "Now the Free Syrian Army are going to get more weapons, more new things which we need strategically on the ground against tanks and against armor. We accept the cease-fire, but that doesn't mean we are not preparing ourselves. Because we don't trust the regime. The regime is going to kill people."
If the cease-fire fails, he said, "we will answer back with huge operations all over the place."
Many world leaders have said the Syrian government is targeting dissidents seeking democracy and the ouster of President Bashar al-Assad, whose family has ruled Syria for 42 years.
The United States is "gravely concerned" over continuing violence, said Susan Rice, the U.S. ambassador to the United Nations, said Monday. If the cease-fire does not hold, she said, that "will call into question the wisdom and viability of sending in the full monitoring presence."
The U.N. Security Council's resolution to send monitors was the group's first on Syria since the crisis in the country broke out more than a year ago.
The Security Council approved the deployment of an advance team of 30 monitors, intended to pave the way for a larger group of as many as 250 observers, and called on Syria to provide them freedom of movement.
The remaining 24 members of the advance team will arrive in Damascus in the coming days, U.N. officials said.
The second, larger deployment is contingent on how the cease-fire holds and whether discussions between Syria and Annan make progress.
U.N. Secretary-General Ban Ki-moon pledged to try to develop by Wednesday a more concrete proposal for an official, and likely more broadly defined, observer mission.
Bouthaina Shaaban, a Syrian government spokeswoman and presidential adviser, said Sunday that the "length of work of the observers and their movement will be determined in coordination with the government of Syria." A protocol agreement on the observers' mission will need to be signed before the larger group is allowed to come, she said, according to state-run Al Dunya TV.
"Syria cannot be responsible for their security unless it participates and coordinates all the steps on the ground," she said.
The United Nations estimates that at least 9,000 people have died since the protests began, while others put the death toll at more than 11,000.