Britain said the suspension Saturday of the United Nations monitoring mission in Syria "calls into serious question" the viability of the entire operation.
"I regret that it has been necessary for the UN mission to decide to suspend patrols and to restrict staff movements due to escalating violence in Syria," Foreign Secretary William Hague said in a statement.
"This underlines the extent of the deterioration of security and stability in Syria, and calls into serious question the viability of the UN mission."
Hague said the worsening situation was directly down to President Bashar al-Assad's regime.
"This worsening situation has been caused by the actions of the Assad regime," he said.
"As (UN-Arab League envoy) Kofi Annan has said, the Syrian regime bears the primary responsibility for ending the violence and I condemn in the strongest terms its absolute failure to do so. I also call on the armed opposition in Syria to stop its violence."
Britain is one of the five permanent, veto-wielding members of the the UN Security Council, which is set to consider the next steps following a briefing Tuesday from Major-General Robert Mood, who heads the 300-strong observer team.
UN observers blamed intensifying violence for the suspension, as troops rained shells down on rebel bastions including Homs, where the opposition warned a massacre was looming.
The unarmed observers have been targeted almost daily since deploying in mid-April to monitor a UN-backed but widely flouted ceasefire.
"This development only underscores the need for the international community urgently to come together to compel the regime to meet its commitments," Hague said.
"The UN Security Council will be considering its options including for the future of the UN mission to Syria in light of a briefing from Major-General Mood on Tuesday."
British Prime Minister David Cameron would have "urgent discussions" about the situation in Syria with other leaders at the G20 meeting at Los Cabos in Mexico on Monday and Tuesday, Hague added.