Weather officials in Thailand are warning of severe storms across the northern region with the start of rainy season this week, after one storm already caused casualties.

Seven people, including four children, were killed when a storm tore off the roof of a primary school's sports center in Phichit Province on Monday.

Students and their parents had taken shelter there from heavy downpours and strong winds. Disaster officials say 18 people were injured.

Prime Minister Prayut Chan-o-cha visited the site on Wednesday. He ordered authorities to speed up their investigation into the cause of the structural failure.