The number of North Koreans slipping illegally into Thailand has surged in recent months, according to immigration bureau officials, as tensions mount on the Korean Peninsula because of Pyongyang's weapons programs.

Thailand is on a popular transit route for North Koreans defecting from the impoverished communist state. Hundreds flee each year to China and make it to Thailand after an overland journey, from where they are usually sent on to South Korea.

In 2016, there were 535 North Korean arrivals in Thailand, but the first six months of this year saw 385 arrivals, according to data from Thailand's immigration bureau seen by Reuters, and more are arriving each week.