(CNN) -- For the next three weeks at least, flying to and within China is going to be a stressful affair, for even the hardiest of travelers.
Hundreds of flights were canceled or delayed Monday and China's civil aviation regulator warned that more flights in 12 airports will be affected over the coming three weeks.
The worst affected airports so far have been Shanghai's two airports -- Pudong and Hongqiao -- two of the country's busiest airline hubs. Another 120 flights were delayed for more than two hours.
Domestic travelers have been switching to high-speed trains (Chinese only) after hearing of the widespread delays, leading to severe overcrowding on main rail routes some have likened to the Chinese New Year migration.
Officials and airlines have shed little light on the reasons for the disruptions but local media have suggested the cause is due to military exercises.
Speculation is rife on social media.
"Perhaps I'm ill-informed but I've never heard of any other countries' military exercises affecting civil airlines on such a large scale," posted user @luoyu on Weibo, China's version of Twitter.
"I've also never seen such vast adjustments affecting a quarter of civil airlines' flights in China's history. Whatever is causing the wide-scale delays, it's making history."
What's going on?
Local officials cited "air traffic restrictions" while Shanghai-based China Eastern Airlines said "the airspace was occupied," according to the Shanghai Daily newspaper.
"Among situations for air traffic restrictions being imposed are when military maneuvers are taking place," the newspaper added.
CCTV, the state television channel, shared on its Weibo account -- China's equivalent of Twitter -- a widely circulated memo, originally published on a Chinese flight crew blog, that said the civil aviation authority has requested airlines to reduce flights by 25% in eastern China.
Hundreds of flights to and from eastern China were canceled or delayed Monday, with 200 flights canceled at Pudong and Hongqiao. Flights departing from other eastern Chinese cities' airports, including Nanjing and Hangzhou, were also affected.
Where to expect delays
"The following 12 airports will experience massive delays for 26 days! Fliers, bring snacks and water!" the CCTV post said.
"From July 20 to August 15, operations of these airports -- including Shanghai Hongqiao, Shanghai Pudong, Nanjing, Hangzhou, Hefei, Jinan, Wuxi, Ningbo, Qingdao, Lianyungang, Zhengzhou and Wuhan -- will be affected by high-frequency exercises conducted by another party. All airlines are requested to reduce flights by 25% and fliers are expected to face delays."
Officials at the Civil Aviation Administration of China declined to comment when approached by CNN.
This is the second large-scale air traffic disruption in China in the last two weeks.
More than 100 flights were canceled or delayed on July 14 in Shanghai.
In December 2006, Shanghai Pudong International Airport closed and hundreds of flights were canceled or delayed due to military exercises.
Two Internet users have been held by police for allegedly fabricating and circulating "anti-corruption-related" claims, according to Hong Kong newspaper South China Morning Post.
The Internet users claimed the disruption was due to the arrest of a high-ranking government official trying to flee the country.
Another 37 people were reprimanded in the crackdown against Internet "rumor mongers," according to SCMP.
China's air traffic woes continue
Lengthy flight delays are common in China.
The country's largest airports, serving the mega-cities of Shanghai and Beijing, suffer some of the worst flight delays in the world.
In June 2013, Beijing's Capital International Airport hit the bottom of a list of the world's most delayed airports, according to U.S.-based air travel information service FlightStats.
Just 18.3% of flights from the airport left on time during that month and around 42% of flights suffered delays of 45 minutes or longer.
Second worst on the list was Shanghai's Hongqiao, where just 24% of flights departed on time.
Overly cautious air traffic control and airspace restrictions for civilian aircraft are two of the reasons for these delays, but flights are also affected by thick smog and once, an unidentified flying object (UFO).
Maggie Hiufu Wong, Steven Jiang, Frances Cha and Daojun Wu contributed to this report.