Chinese internet search giant Baidu Inc posted its first quarterly loss since going public in 2005, and the head of its core search business resigned as the company steps up efforts to increase sales.

The company reported net loss of 327 million yuan ($49 million) in the first quarter of 2019, compared to net income of 6.7 billion yuan in the same period of last year, while its revenues reached 24.1 billion yuan, up 21 percent year-on-year.

Xiang Hailong, who resigned as senior vice-president of the search business, has been replaced by Shen Dou, who was overseeing Baidu's mobile business. Previously Shen served as vice-president of Baidu's mobile products, overseeing the development of Baidu app, Haokan short video app and Baidu smart mini program.

"For senior managers, saying 'I have tried' is not enough and we need to make sure we win in the battlegrounds where we must win," said Robin Li, chairman and CEO of Baidu, in an internal letter.

"Facing the uncertainties of the external environment and changes in the market, we can't wait, we can't put our hope on others and we can't fear," Li said, appealing for telling truth and making innovation.

He added Baidu's mobile foundation continues to strengthen with search-powered AI, and its new AI businesses are making strong progress. In March, Baidu's mobile reach expanded to 1.1 billion monthly active devices, while DuerOS voice assistant installed base reached 275 million devices and generated 2.37 billion monthly voice queries.

"We are leveraging Baidu's artificial intelligence to provide enterprise solutions to businesses and local governments, which significantly expands our total addressable market," he added.

Herman Yu, CFO of Baidu, said: "We anticipate online marketing in the near term to face a challenging environment. We will take this opportunity to improve our monetization capabilities and review our businesses for operational efficiency, while recognizing the importance to invest for sustainable long-term growth."