Thailand deployed military special forces in the northern province of Chiang Rai on Thursday after flooding marooned thousands that authorities are trying to reach with boats and helicopters.

At least 33 people have died across Thailand since mid-August from a spate of rain-related incidents including landslides, with nine fatalities this week in two northern provinces hit by adverse weather brought by Typhoon Yagi, according to the government.

Yagi, the strongest storm to hit Asia this year, has killed at least 197 people in Vietnam, where it made landfall on Saturday and flooded parts of capital city Hanoi.

In Thailand’s northernmost province of Chiang Rai, flood waters had started receding from some areas of the hard-hit Mai Sai district, but many riverside settlements were still flooded, said the head of the district, Narongpol Kid-an.

“We are rushing to bring out people trapped inside their houses,” he told Reuters.

“There are still hundreds of people who need to be rescued.”

Local authorities were using boats and helicopters, with support from Thai Navy SEAL special forces, to conduct rescue operations, he said.

“The situation is very delicate," Narongpol said. "When it rains, the water rises very quickly.”

The Thai Navy said it deployed nine flat-bottom boats to deliver over a thousand aid packages in Chiang Rai.

Flood waters had also entered parts of Chiang Rai City, one of the largest settlements in northern Thailand, inundating key roads and urban areas.

Out of 10 scheduled flights into and out of Chiang Rai’s airport on Thursday, nine had been cancelled, according to its website.

The airport itself had not been flooded but rising waters had made roads to the facility impassable, said Kornchit Chomphudeng, head of Chiang Rai’s Office of Disaster Prevention and Mitigation.

“The flooding this time has spread over a wider area than the recent floods,” he said.

A mountainous region bisected by the Kok River, Chiang Rai was one of five northern Thai provinces hit by severe floods last month.

Authorities were planning to rely on Thai Air Force helicopters to send food and water to some 3,000 people marooned in Chiang Rai’s Ban Kwai Wua Dam area, the provincial office said. Another 200 people were trapped in a school, waiting to be evacuated to a rescue centre.

Thailand’s Office of National Water Resources on Thursday issued a warning to 36 provinces, including the capital Bangkok, for potential flash floods due to heavy rainfall.