The latest development comes just days after the suspicious killing of the North Korean leader Kim Jong-un's half brother, Kim Jong-nam, at an airport in Malaysia.
Kim, who was largely estranged from his family, had spent much of his time overseas in the Chinese territory of Macau, where he was seen to have had the protection of China.

The tightening from China has been incremental but this is a key move. It follows North Korea's test of a more sophisticated missile earlier this month. It may also be linked to the assassination of Kim Jong-un's half brother this week.
For North Korea, this targets the trade it relies on most heavily for cash. Almost all its coal exports go next door to China. Up to now the Beijing authorities had permitted some trade in coal, for "the people's" wellbeing'. That should now end.
China stopped importing some precious metals almost a year ago and banned the sale going the other way of fuel and other items associated with the weapons programme.
South Korea's Yonhap news agency reported last week that the $1m coal shipment had been stopped at Wenzhou port, on China's eastern coast on 13 February.
A day earlier, North Korea had tested an intermediate-range ballistic missile in defiance of UN Security Council resolutions banning the country from carrying out such actions.
It was the first such test since US President Donald Trump took office last month. Prior to his inauguration, Mr Trump said the US should pressure China to get North Korea "under control".
"China has… total control over North Korea," he said in an interview with Fox and Friends on 4 January.
"And China should solve that problem. And if they don't solve the problem, we should make trade very difficult for China.
China last year vowed to halt coal imports to North Korea with some exceptions. Despite this, Chinese data showed that North Korean coal exports to China had risen more than 12% in the past 12 months.