Timothy Savage, a security expert at the Nautilus Institute, said the area where the boat was seized is "not clearly defined" and that "most of these countries in the region have different interpretations of what their exclusive economic zones are".


 
"We frequently have ships from one country or another not just from South Korea or North Korea but even China and Japan crossing into each other's territorial waters, so it wouldnt be surprising for a ship [to] accidentally cross into the North Korean zone," he told Al Jazeera.
 
"Compared to what the North Koreans have been threatening in terms of a physical response, seizing a fishing boat, while it is unfortunate for the fishermen involved, is a relatively low response."

Savage added: "In the old days, the North Koreans would usually keep the people there for some time and try to indoctrinate them into the North Korean way of life.

"They haven't done this for some years now. Now it's more likely they would try to gain some sort of concessions from South Korea before they release them."

The boat was carrying four South Koreans and three Chinese sailors, according to China's Xinhua news agency.

Tensions between the two countries have risen in recent months, since the sinking of a South Korean warship in March, which killed 46 sailors.

A South Korean investigation blamed the incident on North Korea. Pyongyang responded by severing all diplomatic ties with Seoul.

South Korea launched naval drills off its western coast last week, and last month carried out a joint military drill with the US navy. North Korea has threatened "physical retaliation" over the exercises.