BEIJING Yonhap) -- China expressed "concern" on Thursday over North Korea's firing of an artillery shell that happened minutes before a Chinese passenger jet passed through the trajectory of the shell.

   North Korea has fired short-range missiles for days into the East Sea and launched a volley of artillery shells into the sea on Tuesday, in an apparent show of force against ongoing joint military drills between South Korea and the U.S.

   Minutes after North Korea fired an artillery shell into the sea on Tuesday, a China Southern Airlines Co. airplane carrying 220 people, which was heading from Tokyo to the northeastern Chinese city of Shenyang, passed through the airspace, according to South Korea's defense ministry.

   Asked about the incident, China's foreign ministry spokesman Qin Gang told reporters during a regular press briefing that, "I want to stress that we attach great importance to the safety of national civil airlines."

   Nations must take "necessary measures in accordance with international practices to ensure the safety of civilian aircraft and ships in relevant airspace and maritime space" when they conduct military drills, Qin said.

   "China will verify the relevant situation with the relevant party and express concern over that," Qin said.

   The Thursday comments by Qin were compared with a somewhat muted reaction a day ago. When asked about the incident on Wednesday, Qin replied, "We have noted the relevant report."

   "China urges relevant parties to stay calm and exercise restraint, be discreet in words and deeds, avoid a repeat of the scenario where there is a progressive escalation of tensions and jointly maintain the peace and stability of the Korean Peninsula," Qin said Wednesday.

   China is North Korea's key ally and economic benefactor.

   South Korea's defense ministry spokesman Kim Min-seok said the Chinese passenger jet was flying at an altitude of 10 kilometers, while the North Korean shell's peak altitude was 20 kilometers. Kim said the CZ 628 flight passed through the airspace about five minutes after the shell hit the water.

   North Korea fired two short-range missiles on Monday into the East Sea after launching similar missiles last week.

   Kim, the South Korean ministry's spokesman, said the North's firing of artillery shells posed a "grave threat" to a civilian airline and was a violation of international aviation regulations.

   South Korean officials said they notified the Chinese embassy in Seoul of the Tuesday incident.