An internal security tool flagged the possible breach beginning on September 8 and later discovered the hackers had accessed customer information and attempted to remove it in encrypted form, the company said.
The hack into Marriott's Starwood is part of Chinese intelligence-gathering efforts that also hacked into health insurers and security clearance files of millions of Americans, The New York Times reports. The MSS, an intelligence and security agency, has been behind numerous Chinese government intrusions into sensitive USA networks in recent years.
The revelation comes amid tensions between on-going tensions between the U.S. and China. Private investigators also identified the techniques as those previously used in attacks attributed to Chinese hackers, Reuters reported.
The discovery comes as US President Donald Trump's administration is planning actions targeting China's trade, cyber and economic policies.
According to Business Insider, for example, the Trump administration plans to declassify United States intelligence reports that reveal China's efforts to build a database with the names of USA government officials with security clearances.
The paper goes on to say " other options include an executive order meant to make it harder for Chinese companies to obtain critical components for telecommunications equipment, a senior American official with knowledge of the plans said".
The Trump administration has been planning to declassify U.S. intelligence reports that show China's efforts to build a database with the names of USA government officials with security clearances, the Times reported. Chinese officials denied knowing anything about the hacking being done on behalf of any government agency, according to the Times.
Last week Canada arrested an executive of Huawei, China's leading telecommunications company, at the request of the United States, which plans to charge her with fraud charges related to sanctions-breaking business dealings with Iran.
The spies stole passport numbers of up to 327 million people - many of whom stayed at Sheraton, Westin and W hotels and at other Starwood-branded properties.
Marriott on Tuesday reiterated its previous comment on the data breach, saying in a statement: "Our primary objectives in this investigation are figuring out what occurred and how we can best help our guests".
In a hack dating back to 2014, hackers gained access to a swath of personal data from Marriott and Starwood hotel guests, including credit card information and passport numbers.