UN Secretary-General Antonio Guterres warned Friday of a looming humanitarian catastrophe in western Myanmar and urged the country's security forces to show restraint after 400 people -- most of them Rohingya Muslims -- died in communal violence.

"The Secretary-General is deeply concerned by the reports of excesses during the security operations conducted by Myanmar's security forces in Rakhine State and urges restraint and calm to avoid a humanitarian catastrophe," said a spokesman for the UN chief.

The ethnic violence has sent tens of thousands of Rohingya fleeing across the border into Bangladesh, while scores of desperate people have drowned trying to cross a border river in makeshift boats