Heavy rains have lashed Vietnam's middle belt in recent weeks, flooding historic sites and popular holiday destinations and causing hundreds of millions of dollars in damage.

Authorities ordered boats to shore and diverted dozens of flights as Koto whipped up huge waves and dangerous winds, state media reported.

Two vessels sank in the rough seas, a fishing boat in Khanh Hoa province and a smaller raft in Lam Dong, according to the environment ministry. It said a total of three people were killed and authorities were still searching for a fourth.

Koto was more than 300 kilometres (185 miles) offshore Sunday morning, having been downgraded from a typhoon to a tropical storm.

Vietnam's weather bureau said it was moving slowly and expected to weaken further before colliding with the coast next week.

It forecast rains of up to 150 millimetres (six inches) on Tuesday and Wednesday from Hue to Khanh Hoa -- regions that have only just recovered from historic floods.

Vietnam is in one of the most active tropical cyclone regions on Earth and is typically affected by 10 typhoons or storms a year, but Koto is the 15th of 2025.

Natural disasters have left more than 400 people dead or missing this year in Vietnam and caused more than $3 billion in damage, according to the national statistics office.

The Southeast Asian nation is prone to heavy rain between June and September, but scientists have identified a pattern of human-driven climate change making extreme weather more frequent and destructive.