Facebook has refused to rule out returning to China despite its commitment to upholding free expression and user privacy.

In a letter to the US senate's intelligence committee, the company said that human rights would be "carefully considered" before it made any further attempt to operate under the authoritarian regime.

But it stopped short of saying that China's extensive censorship and surveillance apparatus currently conflict with its principles, or that it would refuse to offer any service there until conditions changed.

"Facebook has committed to freedom of expression and privacy standards... and we are independently assessed on our compliance on a biennial basis," the company said.

"In keeping with these commitments, rigorous human rights due diligence and careful consideration of free expression and privacy implications would constitute important components of any decision on entering China."

But, it added, "no decisions have been made around the conditions under which any possible future service might be offered in China."