(Recasts lead, adds quotes from China Southern Airlines chairman)
BEIJING, March 7 (Reuters) - China has expressed its "deep concern" to North Korea after a Chinese airplane crossed the path of a rocket launched by the isolated state, China's foreign ministry said on Friday.
On Thursday, South Korea's defence ministry said a Chinese civilian plane had "passed as the ballistic missile (from North Korea) was in the course of descending".
The plane was flying from Tokyo's Narita airport to the northeastern Chinese city of Shenyang on Tuesday, the ministry said.
"On this issue, we have already contacted the North Korean side to convey our deep concern," Chinese Foreign Ministry spokesman Qin Gang told a daily news briefing.
China is North Korea's most important diplomatic and economic supporter, though Beijing's patience with Pyongyang has been severely tested following three nuclear tests and numerous bouts of saber rattling, including missile launches.
On Thursday, Qin had said the Chinese airplane was unaffected and urged Pyongyang to ensure the safety of civilian aircraft.
Media reports had identified the Chinese airplane as a China Southern Airlines flight.
Company chairman Si Xianmin confirmed to Reuters that one of his firm's planes intersected the trajectory of the North Korean missile.
"The report is true, but the missile was far away from the plane -- it was six minutes away and I don't know how many kilometres away in terms of altitude," Si told Reuters.
A spokeswoman from the Civil Aviation Administration of China told Reuters on Thursday that she had read the reports but had no knowledge of the incident, adding that the agency had not been contacted by China Southern Airlines. (Reporting by Ben Blanchard and Fang Yan in BEIJING and Jack Kim in SEOUL; Writing by Sui-Lee Wee; Editing by Clarence Fernandez)