A United Nations (UN) chief has warned Myanmar is "heading towards war" as hundreds of people have been killed by the military coup since February.

The reported death toll marks the biggest one-day total for a single city since March 14, when just over 100 people were killed in Yangon.

At least 82 people have been killed in a crackdown by Myanmar security forces, with reports suggesting grenades and mortars were used on demonstrators.