* Organiser says some Chinese unable to attend youth event
* N. Korea fires 3 rockets shortly before pope's arrival in Seoul
* Pope's predecessor had been forced to fly around China (Adds comments from Catholic activists, further N. Korea projectile launch)
By Philip Pullella and Ju-min Park
SEOUL, Aug 14 (Reuters) - Pope Francis sent an unprecedented message of good will to China on Thursday before touching down in Seoul, but the first papal trip to Asia in 15 years got off to a shaky start with the news some Chinese had been barred from joining a youth celebration.
About half of more than 100 Chinese who had planned to attend an Asian Youth Day event during the pope's visit are unable to attend due to "a complicated situation inside China", Heo Young-yeop, spokesman for the Committee for the Papal Visit to Korea, told reporters.
He declined to give further details, citing their safety. Another organiser, who declined to be identified, said some of the would-be attendees had been arrested by Chinese authorities.
Beijing rejects Vatican authority over its Catholics.
China's Foreign Ministry did not immediately respond to requests for comments either on the Pope's good will message or the Chinese who were barred from attending the youth event.
As the pope's plane approached South Korean airspace off the west of the peninsula, North Korea test fired three short-range rockets into the sea off its east coast, according to South Korea's defence ministry.
The test site was hundreds of kilometres away from the pope's plane. North Korea fired two more projectiles from the same location early on Thursday afternoon.
The North has tested an unprecedented number of rockets and missiles this year and has in recent weeks said the launches were in retaliation to U.S.-South Korean military exercises scheduled to start on Monday.
Pyongyang often stages such tests when rival South Korea is in the global spotlight, in what is seen as a means to grab a share of the attention.
The Argentine pope will spend five days in South Korea, meeting some of the country's five million Catholics on the first trip by a pontiff to Asia since 1999, but much of the attention will be on the Vatican's relations with China.
"Upon entering Chinese air space, I extend best wishes to your Excellency and your fellow citizens and I invoke the divine blessing of peace and well-being upon the nation," he said in a radio message to President Xi Jinping.
It was the first time a pope had been allowed to fly over China on Asian tours. His predecessor John Paul II had to avoid Chinese airspace because of the fraught relations between Beijing and the Vatican.
The Vatican has had no formal relations with China since shortly after the Communist Party took power in 1949. The Catholic Church in China is divided into two communities: an "official" Church known as the "Patriotic Association" answerable to the Party, and an underground Church that swears allegiance only to the pope in Rome.
Ren Dahai, director of Jinde Charities, a Catholic charity in China, said he had heard that some Chinese Catholics had been restricted from traveling to South Korea and some had been allowed to go, and that the matter had been a hot topic of discussion among Chinese Catholics in the last two days.
Some of the travelers had been stopped at airports and prevented from passing through customs, Ren said he had heard.
"This seems to me like it's dependent on the local government and the ideas and practices of local officials," Ren told Reuters by telephone from northern Hebei province, a Catholic stronghold.
"Some places will be stricter, while some will be looser."
Anthony Lam, a senior researcher at the Holy Spirit Study Centre in Hong Kong, an organ of the Diocese of Hong Kong, said Chinese authorities had warned more than 100 Chinese Catholics not to go to Seoul but said he was not aware of any arrests.
"Local officials will tell them: 'You should cooperate with us and not go to Seoul this time. Otherwise, you would know the costs'," Lam said in a phone interview. He said he had the information from "different channels" but declined to specify further.
"At least 10 days ago, or two weeks, the government started lobbying them not to go and later on gave them warnings."
NORTH KOREA CRACKDOWN
North Korea had turned down an invitation from the South Korean Catholic church for members of its state-run Korean Catholic Association to attend a papal mass on Monday in Seoul, citing the start of the joint U.S.-South Korean military drills.
The two Koreas have been divided since the 1950-53 Korean war, which left millions of families separated.
Missionaries and aid groups have described a crackdown by Chinese authorities on Christian charity groups near its border with North Korea in recent months. The sweep along the frontier is believed to be aimed at closing off support to North Koreans who flee persecution and poverty in their homeland and illegally enter China before going on to other nations, usually ending up in South Korea.
China is North Korea's main ally. (Additional reporting by James Pearson in Seoul, Ben Blanchard and Sui-Lee Wee in Beijing; Editing by Tony Munroe and Jeremy Laurence)