United Nations observers in Syria have suspended their mission, saying the conflict has shifted in the past ten days, becoming too dangerous for them to monitor.

Halfway through a three-month mandate from the UN Security Council to report on adherence to a UN-brokered peace plan, the head of the mission said neither side was prepared to pull back from the fighting.

"The lack of willingness by the parties to seek a peaceful transition and the push toward advancing military positions is increasing the losses on both sides – men, women and children are being killed every day," said Major General Robert Mood in announcing the observers would remain on base until further notice.

He said the fighting put the unarmed observers at significant risk.

Mood said in Damascus on Friday that some of the member states that provided unarmed observers were unwilling to continue to expose them to danger.

The mission had perhaps been dealt a deathly blow with images of monitors entering the deserted town of al-Haffeh, which they had been unable to enter, to find evidence of dozens of people killed.

In a briefing to reporters, Mood said opposition forces had become more effective in inflicting losses on government security forces.

He said the Syrian army appeared to be losing, rather than gaining, control of some areas, making the conflict even more volatile.

"When we go into these areas we are leaving areas that are in control of government forces," Mood said.

"We do not see a situation in which the control of these areas are getting bigger – rather a situation where the control of these areas is being challenged."

Syrian security forces have responded with increasingly heavier weapons, turning a conflict that began as a popular uprising, suppressed with riot police and rifles, into one where entire neighbourhoods are being shelled.

"The employment of artillery, mortars, army formations has become more of what you would characterise as a classic use of armed forces," said the Norwegian general.

Mood said efforts to stop the violence were complicated by a lack of support inside Syria for the external opposition, and a lack of cohesive leadership among the armed opposition.

In the next few days, Mood is to brief the UN Security Council, which will then decide whether to take a further step.

"I think what we'll see is the security council will remain deadlocked," said Salman Shaikh, an analyst at the Brookings Doha Centre.

He said while some security council members would push for UN authorisation for the use of force to stop the fighting, Russia would continue to oppose it.