Thirteen citizens of the Democratic People's Republic of Korea left China last week in the early hours with valid passports, the Foreign Ministry said on Monday in response to reports of DPRK restaurant employees leaving for Seoul.

"Chinese public security authorities have been informed of the case about missing DPRK citizens in China recently," said ministry spokesman Lu Kang.

He said an investigation found that the DPRK citizens left China on Wednesday in a normal way with valid passports.

"I'd like to stress that ... they did not enter China illegally."

He added that in cases in which citizens from the DPRK enter China illegally, Beijing's stance is to properly handle the cases according to international and domestic laws and humanitarian principles.

Authorities in Seoul said on Friday that 13 employees of a DPRK state-run restaurant in Ningbo, Zhejiang province, left together and arrived in the Republic of Korea.

People at the restaurant later told Seoul-based KBS TV that the employees - one male manager and a dozen women - disappeared on the night of April 5. The restaurant has ceased operation.

A source who requested anonymity told Seoul-based Yonhap News Agency on Monday that the employees flew to Thailand and then went on to Laos, and finally flew to Seoul on Thursday.

It was reported that they got help from the ROK government after arriving in Thailand.

Yonhap quoted an official with the Unification Ministry in Seoul as saying that the employees chose to go to the ROK because the DPRK faces harsh international sanctions due to its nuclear test on Jan 6 and rocket launch on Feb 7.

The official said the employees also were under growing pressure to send foreign currency back to the DPRK after the sanctions took effect, and were afraid of punishment if they failed to send the required amount.

"In this case, the departure of the DPRK people from China has fully followed the legal procedures," said Zha Daojiong, a professor of international relations and security at the School of International Studies at Peking University.

"They have valid passports, and the Chinese customs officers could not predict their aim. Besides, China is not even a transit state in this case."

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