Air Koryo, North Korea's flagship airline, has added flights to its regular service between Pyongyang and the northeastern Chinese city of Shenyang, sources familiar with the North said Tuesday.

Air Koryo has been known to fly between the two cities once every Wednesday and Saturday, but it now operates two flights on each of those days, according to an aid group official who traveled to the North Korean capital from Shenyang last week.

The official, who spoke on the condition of anonymity, said he spotted many Chinese and other foreigners at his hotel in Pyongyang and across the city.

It appears that the number of flights between Shenyang and Pyongyang was increased because of the Arirang Festival, which started on Aug. 1, the official said.

The festival, named after a famous Korean folk song, has been held almost annually since 2002, when it debuted to celebrate the 90th birthday of North Korea's late founder, Kim Il-sung. It is believed to be the largest gymnastics show in the world, with synchronized acrobatics, dances and flip-card mosaic animations.

Another aid group official who traveled to Pyongyang on the same route two months ago said there were no additional flights in place at the time, adding that such temporary expansions were not unusual during peak seasons for visitors.

Other North Korea watchers, however, said the new flights may be a permanent measure aimed at facilitating growing economic ties between the communist allies.

It may be a measure to accommodate a growing number of Chinese people traveling to Pyongyang (on business), as the two nations are known to be actively taking part in economic cooperation, said a North Korea expert, who spoke on the condition of anonymity.

Cho Bong-hyun, a researcher at the IBK Economic Research Institute, said he confirmed the expansion of the flight service, although he could not be sure whether it was permanent.

It seems that the number of flights was increased due to the Arirang Festival and the larger number of Chinese people traveling to North Korea for joint economic projects, he said.

The festival is set to run until Sep. 9.