Syrian forces have blocked UN monitors from entering two villages where 86 people, including women and children, were allegedly massacred by pro-government militiamen, the mission commander has said.
General Robert Mood said observers were being turned back by the Syrian army and some UN patrols had been stopped by civilians in their efforts to enter the villages of Qubair and Maarzaf.
Many of the victims were stabbed or shot at close range.
"They are being stopped at Syrian army checkpoints and in some cases turned back," he said.
His comments came after Prime Minister David Cameron condemned the "brutal and sickening" massacre and called for "concerted action" from the international community.
US Secretary of State Hillary Clinton said the violence was "unconscionable".
The UN has 297 unarmed observers in Syria to verify the implementation of a peace plan negotiated by UN-Arab League envoy Kofi Annan, which includes a ceasefire that supposedly came into force in mid-April.
General Moody's claims came hours before a divided UN Security Council convenes to review the crisis.