The number of UN observers already on ground in Syria has risen to 189, most of whom deployed in restive Syrian cities, in the hope that their presence would calm the simmering tension in the unrest-ridden country, sources told Xinhua.

Sources said that six new observers joined Sunday four of their comrades in northern Aleppo province, making the Aleppo team of 10 observers.

Another team of observers visited Sunday the central province of Hama and met with its governor, Anas Na'em, during which the governor stressed his resolution to facilitate the monitors' work, urging them to stick to evenhandedness and precision when documenting what they see on ground.

The observers arrived in Syria last month to monitor the implementation of UN-Arab League joint special envoy Kofi Annan's six-point peace plan that calls mainly for the halt of all violence by both sides of the conflict in order to pave the way for a political settlement to Syria's 14-month unrest.

However, since their arrivals, the violence rate has notably increased, which cast doubts on the prospect of the mission.

The Syrian opposition has shown thin patience to the observer mission, saying that the observers "have become perjurers."

The UN observers have been attacked twice since their arrival, which technically made them part of the conflict rather than the solution. On Saturday, the European Union sent 25 armored vehicle to the UN monitor mission in Syria to ensure the safety of the observers.

Annan has recently said that a total of 300 observers will be on ground in Syria by the end of May.

On April 21, the UN Security Council unanimously adopted a resolution to approve the deployment to Syria of up to 300 unarmed UN military observers in order to monitor a fragile ceasefire between the Syrian government forces and armed opposition fighters.

Under the resolution, the UN Supervision Mission in Syria has an initial period of 90 days to carry out its mandate.

Meanwhile, the Britain-based Syrian Observatory for Human Rights said that Syrian forces killed five people in the village of al-Tamana, about 55 km northwest of Hama. However, activists' account could not be independently verified.

The state-run SANA news agency said that a law-enforcement member was killed by the gunfire of an "armed terrorist group" in Hawaij Ziban village near eastern Deir al-Zour province.

Quoting a source in the area, SANA said that the armed group attacked a patrol of law-enforcement forces in the village, killing one of them. It added that the authorities clashed with the gunmen and killed two of them.

The Syrian government blames the unrest on armed groups and extremists backed by a foreign plot. It said that more than 2,500 army personnel had been killed during the turmoil.

On the other hand, the UN said more than 9,000 people had been killed over the past year, attributing the hefty toll to the regime's alleged bloody crackdown.