Violence in Myanmar has spiralled into the "the world's fastest-developing refugee emergency" and a "humanitarian nightmare", the UN chief has warned.
Secretary General Antonio Guterres called on Myanmar to end its military operation, which has sparked the exodus of over 500,000 Rohingyas since August.
He also demanded "unfettered access" to the region to deliver humanitarian aid.
Earlier, at least 14 Rohingyas, all women and children, drowned after their boat capsized off Bangladesh's coast.
Survivors say the boat overturned after apparently hitting a submerged object near the coastal city of Cox's Bazar.
In the past 48 hours, about 2,000 Rohingyas have arrived in Bangladesh by boat, fleeing violence in Myanmar's Rakhine state.
In Thursday's briefing to the UN Security Council, Mr Guterres said: "The situation has spiralled into the world's fastest developing refugee emergency, a humanitarian and human rights nightmare."
"We have received bone-chilling accounts from those who fled - mainly women, children and the elderly.
"These testimonials point to excessive violence and serious violations of human rights, including indiscriminate firing of weapons, the use of landmines against civilians and sexual violence."
Mr Guterres also warned that "the failure to address the systematic violence could result in a spill over into central Rakhine where an additional 250,000 Muslims could potentially face displacement".
UN aid personnel were forced to leave Rakhine when the military began a crackdown on Rohingya militants behind attacks on security personnel in August.
Fleeing Rohingyas - the majority of whom are Muslim - accuse Myanmar's military, backed by Buddhist mobs, of trying to drive them out with a campaign of beatings, killings and village burnings.
Images and reports from journalists confirm many villages have been razed.
But the military say they are targeting only militants.