Opinion polls published on Sunday in the United States and Germany cast doubt over the level of trust people have in Facebook over privacy, as the firm ran advertisements in British and U.S. newspapers apologizing to users.

Fewer than half of Americans trust Facebook to obey U.S. privacy laws, according to a Reuters/Ipsos poll released on Sunday, while a survey published by Bild am Sonntag, Germany's largest-selling Sunday paper, found 60 percent of Germans fear that Facebook and other social networks are having a negative impact on democracy.

Facebook founder and chief executive Mark Zuckerberg apologized for "a breach of trust" in advertisements placed in papers including the Observer in Britain and The New York Times, Washington Post, and The Wall Street Journal.

"We have a responsibility to protect your information. If we can't, we don't deserve it," said the advertisement, which appeared in plain text on a white background with a tiny Facebook logo.

The world's largest social media network is coming under growing government scrutiny in Europe and the United States, and is trying to repair its reputation among users, advertisers, lawmakers and investors.

This follows allegations that the British consultancy Cambridge Analytica improperly gained access to users' information to build profiles of American voters that were later used to help elect U.S. President Donald Trump in 2016.

U.S. Senator Mark Warner, the top Democrat on the Senate Intelligence Committee, said in an interview on NBC's Meet the Press" on Sunday that Facebook had not been "fully forthcoming" over how Cambridge Analytica had used Facebook data.

Warner repeated calls for Zuckerberg to testify in person before U.S. lawmakers, saying Facebook and other internet companies had been reluctant to confront "the dark underbelly of social media" and how it can be manipulated.