He takes poison before he can be questioned on the reasons for his escape, a North Korean source says.
Sources say Mongolia will comply with UN resolutions to avoid driving away foreign investment.
Dash through fog marks the 15th North Korea this year to defect directly to the south across the land border or by sea.
Pressure to earn foreign currency for Pyongyang drives the market in banned wildlife products.
The move reflects growing tensions, as China joins in with international sanctions trying to pressure Pyongyang.
Countries in region are committed to upholding U.N. Security Council resolutions, senior American diplomat says.
Move is seen as an effort to make it easier to import or smuggle goods from China as sanctions bite.
Top American diplomat says US open “to talk anytime” North wants, but White House says time not ripe.
Thefts of devices holding sensitive information prompt the government order, issued in September.
Finding life difficult in South Korea, they go back to North Korea and identify others with contacts in the South.
Angered by reports of wrongdoing, Kim Jong Un cracks down on wealthy North Koreans with ties to government officials.
They work out of newly opened 'branches' of state-linked firms in an effort to increase trade volume in the sanctions-hit country.