Reuters reports: U.N. monitors came under fire in Syria on Thursday while trying to investigate reports of a new massacre that raised the pressure on world powers struggling to halt the carnage and save a peace plan from collapse.

U.N. Secretary-General Ban Ki-moon described as “unspeakable barbarity” the reported killing of at least 78 villagers by forces loyal to Syrian President Bashar al-Assad and warned that a civil war was imminent.

Speaking at a special session of the U.N. General Assembly on Syria, international envoy Kofi Annan acknowledged his U.N.-Arab League peace plan was not working and said there must be “consequences” for those who do not comply.

Ban said hopes for consolidating the peace plan were fading and Annan himself warned the U.N. Security Council that the crisis in Syria could soon spiral out of control, diplomats said. Annan, Ban’s predecessor as U.N. secretary-general, called for “substantial pressure” on Damascus to stop the violence.