“Protecting data privacy has to be a higher-tier issue in terms of national security,” Wyden said. “The prospect of data that is shared with shady businesses, hackers and foreign governments is a massive privacy and a national security concern.”

Sandberg responded that the company “can’t commit to make [details of the relationship] public because a lot of it has sensitive information”. But she said Facebook “will prioritise the request” and get back to the lawmakers “as quickly as possible”.

When Republican Senator Marco Rubio of Florida said he worried that technology companies in the US were kowtowing to authoritarian regimes globally to access their markets, Sandberg replied that the tech firm “would only operate in a country where we can do so in keeping with our values” and “that will apply to China as well”.

The new requests from lawmakers about US technology giants’ businesses relationship with China come amid rising concern about Chinese technology companies engaging in espionage activities against the US.

They also follow Washington’s move to ban government entities from buying Huawei and ZTE products under legislation signed last month by US President Donald Trump.

Silicon Valley’s ties with Beijing also further heighten trade tensions between the world’s two largest economies, partly triggered by US wariness that China is stealing cutting-edge technologies as it seeks to become a dominant technology player.

It is known that Alphabet, parent of Google, has strategic partnerships with Chinese mobile device makers Huawei and Xiaomi, as well as with the Chinese technology platform Tencent.

In June, Facebook disclosed that its partnerships with Chinese telecom companies such as Huawei allow the Asian firms to access Facebook users’ non-public data.

Democratic Senator Mark Warner of Virginia, vice chair of the Senate Select Committee on Intelligence, asked Google and Twitter in June for information about the data sharing agreement the companies have with their Chinese vendors.

Warner also asked Facebook to provide information about those partnerships and other agreements into which Alphabet may have entered with third-party vendors based in China. A similar request was also sent to Twitter.

Warner said in the letter that “the relationship between the Chinese Communist Party and equipment makers like Huawei and ZTE has been an area of national security concern”.

Alphabet was also invited to testify on Wednesday but the company declined to take part - a pointed noted by lawmakers.

Rubio began his remarks by pointing to the “empty seat of Google” and noting the company “is not here today maybe because they are arrogant”.

Warner said, "I'm deeply disappointed that Google, one of the most influential digital platforms in the world, chose not to send its own top corporate leadership to engage this committee.”

Google in August said it planned to build a search engine that would block certain search terms and leave out content blacklisted by the Chinese government, in an effort to reenter that market. The pushback sparked by the announcement has led to more than 1,000 Google employees signing a letter in protest

Watch: Facebook uncovers ‘coordinated’ campaign to disrupt US midterm elections

In other news from the hearing, Sandberg acknowledged to the committee that the company was too slow to respond to Russian efforts to interfere in the 2016 US election and general American political discourse, but insisted it is doing better“

“We’ve removed hundreds of pages and accounts involved in coordinated inauthentic behaviour – meaning they misled others about who they were and what they were doing,” Sandberg said.

“When bad actors try to use our site, we will block them,” she said.

Dorsey also described Twitter’s tighter monitoring of malicious use of its platform, including notifying law enforcement last month of accounts that appeared to be located in Iran. He said it suspended 770 account for violating Twitter policies.

Facebook, Twitter and other technology firms have been on the defensive for many months over political influence activity on their sites as well as concerns over user privacy.

Before the hearing, Trump, without appearing to offer any evidence, accused the companies themselves of interfering in the US midterm elections in November, telling The Daily Caller that social media firms are “super liberal”.

Trump told the conservative news outlet in an interview conducted on Tuesday that “I think they already have” interfered in the November 6 election. The report gave no other details.

As the hearing began, social media stocks fell, with Twitter down 4.5 per cent and Facebook around 1 per cent lower.

Executives from the companies, which have repeatedly denied political bias, have travelled to Washington several times to testify in Congress, including 10 hours of questioning of Facebook CEO Mark Zuckerberg over two days in April.

Dorsey was to follow his Senate testimony with an appearance at an afternoon hearing looking at that issue in the House of Representatives.

Dorsey was to tell the House Energy and Commerce Committee that Twitter “does not use political ideology to make any decisions”, according to written testimony made public on Tuesday.

Trump faulted Twitter on July 26, without citing any evidence, for limiting the visibility of prominent Republicans through a practice known as shadow banning.

Last week Trump accused Google’s search engine of promoting negative news articles and hiding “fair media” coverage of him, vowing to address the situation without providing evidence or giving details of action he might take.

Republicans control majorities in both the Senate and House, but the House’s approach to the election issue has been far more partisan than in the Senate.

Additional reporting by Reuters