A roadside bomb struck a car belonging to the observer mission minutes after the shooting began. The activists said the observers were not among the injured.
"This is a real massacre and it took place in the presence of UN observers," said Rami Abdul Rahman, the head of the Britain-based Syrian Observatory for Human Rights. He called for an international investigation and for the monitors to state what they saw.
Ahmad Fawzi, a spokesman for the UN-Arab League special envoy to Syria, Kofi Annan, confirmed the observers were caught up in the country's violence as they met with the rebel Free Syrian Army.
"The UN mission in Syria reports that shortly after 2pm local time today, a [UN] convoy of four vehicles was struck by an explosion from an improvised-explosive device," Mr Fawzi said.
Meanwhile, the Paris-based president of Syria's exiled opposition won reelection to another three-month term. Burhan Ghalioun was chosen to continue to head the Syrian National Council (SNC) by a vote of the group's general secretariat in Rome, where Mr Ghalioun and other SNC members were meeting.
Mr Ghalioun won despite far-reaching criticism of his mismanagement of the group since its creation in August.
Backed by Gulf states and France, the academic is viewed by many as a consensus figure. But Mr Ghalioun's new term got off to a difficult start when his group's frustration with the Arab League forced the postponement of a meeting between the two in Cairo.
Officials from the 310-member council declined to attend the meeting that was scheduled to take place today and tomorrow because the Arab League has refused to recognise the SNC as an official organisation.
The bickering comes after Saudi Arabia's foreign minister, Prince Saud Al Faisal, warned a month-old peace plan overseen by the Mr Annan, had failed to stop fighting in Syria.
Confidence in the plan "has started to decrease quickly" because "violence is still continuing", Prince Faisal said after a gathering of Arab leaders in Riyadh on Monday, adding that "nobody is satisfied" with the plan.
Saudi Arabia, along with other Gulf countries and the United States, has supported the Annan plan despite their insistence that ending the violence should begin with the departure of Syria's president, Bashar Al Assad.
Evidence suggests the uprising may be morphing into a civil war, greatly influenced by foreign fighters. Foreign Islamists, many from Tunisia, have reportedly been flowing into Syria to fight government forces.
* With additional reporting by the Associated Press, Agence France-Presse and Reuters