A convoy of UN truce observers came under bomb attack in a Syrian town on Tuesday during a funeral procession in which a monitoring group said regime forces "massacred" 20 people.
The incidents took place as Syria's anti-regime revolt entered a 15th month of relentless violence that has killed more than 12,000 people and amid growing fears that a UN-backed peace plan will fail.
The blast occurred as the convoy made its way along a narrow street in Khan Sheikhun, a town in Idlib province, said activists, rebels and a watchdog.
UN spokesman Martin Nesirky said the bomb exploded in front of the convoy and that three vehicles were damaged but no casualties were reported.
Ahmad Fawzi, spokesman for UN-Arab League envoy Kofi Annan, said the UN supervisory mission had sent a patrol to help the stranded monitors.
The Syrian Observatory for Human Rights said 26 people other people were killed around the country.
"The Syrian regime committed a massacre Tuesday during a visit by UN monitors to Khan Sheikhun," in the northwestern province, said the Britain-based watchdog.
It was the second roadside bombing involving the observers' vehicles in less than a week, after a convoy in the flashpoint southern city of Daraa wounded six Syrian soldiers on Wednesday.
Video uploaded to YouTube by activists showed a convoy of UN vehicles surrounded by dozens of people before a blast was heard and a puff of smoke went up in front of the leading UN-marked jeep.
It was unclear from the footage if there were any casualties, and the vehicle drove away despite damage to its hood.
The authenticity of the video could not be verified.
Activists said the UN convoy had come under attack and one car was hit by a shell, prompting the monitors to quickly leave the area.
Major Sami al-Kurdi, a spokesman for the rebel Free Syrian Army, told AFP the monitors had arrived during the funeral and that their presence had encouraged more mourners to turn out and join the procession.
"The regime dared to attack the procession, however, and then targeted the vehicles of the UN observers from a regime checkpoint," he said.
The United Nations reaffirmed its condemnation of any violence against the monitors.
"This mission is there to help the people of Syria, to help ensure that the six-point plan is implemented," Nesirky said, referring to Annan's peace plan. "Anything that interrupts their work and endangers the lives of UN personnel is something we would condemn."
The latest bloodshed comes despite a truce brokered by Annan as part of a six-point plan aimed at ending violence that has swept Syria since March 2011, when the uprising against President Bashar al-Assad broke out.
The United Nations has accused both sides to the conflict of violating the ceasefire and warned Syria was edging closer to full-blown civil war.
Damascus maintains that foreign-backed "armed terrorist groups" are behind the unrest, trying to undermine the regime and scuttle attempts at political reform.
Meanwhile, Annan urged Syria to stop delaying an agreement on allowing UN access to more than one million Syrians in need of assistance, saying the process had been "very slow."
Annan "remains extremely concerned about the plight of one million Syrians in need of humanitarian assistance," said UN spokesman Martin Nesirky.
In Riyadh, Saudi Arabia's foreign minister warned on Monday that confidence in Annan's peace mission was fading fast because of the bloodshed.
Part of Annan's plan includes the deployment in flashpoint areas of about 300 UN military observers, and their number reached more than 200 on Tuesday, said the mission's chief, Major General Robert Mood.
Although the number of casualties has decreased since the observers began trickling into Syria in mid-April, the violence has not stopped.
The Observatory says more than 12,000 people, the majority of them civilians, have died since the uprising began on March 15 last year, including more than 900 killed since the April 12 truce.
In addition, foreign doctors returning from a secret mission inside Syria reported people wounded in the regime's crackdown on dissent, as well as the medics who treat them, risk arrest and even torture.
"The aim of the Syrian army was clearly to kill the wounded and those suspected of treating them," said one doctor on the team from Doctors Without Borders (Medecins Sans Frontieres), which entered Syria illegally after failing to get government permission.
In neighbouring Lebanon, the army deployed in sectors of the northern city of Tripoli after nine people were killed in three days of sectarian violence between groups who support and oppose Syria's regime.
Troops entered Syria Street, the frontline of fighting between a majority Sunni Muslim district and another that is mostly Alawite, an offshoot of Islam to which Assad belongs.
In a related development, a Lebanese farmer was killed by a rocket as he worked his fields on the Syrian side of the border, Lebanese state news agency ANI said.