BEIJING—China's largest online travel company, Ctrip.com International Ltd., said Thursday that some of its servers had been hacked, causing disruptions to its website and mobile app services.

The Nasdaq-listed company said that it suspected some of its servers had been attacked by "unidentified sources" shortly after 11 a.m. local time, but that Ctrip's data and customer reservation data remained intact.

More Chinese are traveling both at home and abroad. By the end of this decade, the number of tourists venturing outside mainland China is expected to double to 200 million, according to estimates by Hong Kong brokerage CLSA.

Booking travel online, especially through the use of mobile apps, is also growing in popularity although gauging online-travel market share in China is difficult because travel companies work in different ways.

In 2013, the government-affiliated China Internet Network Information Center estimated market share of about 34% for Ctrip and 22% for Qunar Cayman Islands Ltd.

China's government regularly denounces malicious hacking, while saying the country and its citizens are among the biggest victims.

In a statement on its verified Weibo microblog account several hours after the disruption began, Shanghai-based Ctrip said it was still investigating the cause of the problem and trying to restore service.

An attempt to use the Ctrip mobile app around 2 p.m. to make a hotel booking yielded a listing of available hotels as normal. However, filters such as hotel rating or location failed to work properly when applied at that time.

U.S.-based Priceline Group Inc. said this week it planned to increase its investment in the Chinese company, investing $250 million through a convertible bond after making a $500 million investment last summer. That could ultimately give Priceline about 10.5% of Ctrip's shares outstanding.

Earlier this month, Expedia Inc. sold its 62.4% majority stake in eLong Inc., a Chinese mobile and online travel service, to a group of buyers that included Ctrip, which said it acquired a 37.6% stake in eLong for about $400 million.

-Liyan Qi, Lilian Lin and Colum Murphy

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