Lawmakers on the House Judiciary Committee's antitrust panel pressed an executive from Amazon.com Inc on Tuesday about allegations that it competed against its own sellers and pushed them to buy advertising and fulfilment services.

Legislators also demanded explanations from Apple Inc about charges for apps and in-app purchases, Facebook Inc for its rapidly changing privacy policy and Alphabet's Google over whether its rivals are demoted in search results.

The committee does not have authority to punish the companies, and any effort to change antitrust laws affecting tech firms would face hurdles in the Republican-controlled Senate. So the questioning served largely to convey the panel's displeasure over many of the companies' business practices.

Representative David Cicilline, chair of the antitrust committee, pressed Nate Sutton, an associate general counsel at Amazon, about allegations that the online retailer used data about independent sellers on its platform to develop products to sell, thus competing against its own sellers.

"You said we do not consult data to compete with other sellers online. You do collect enormous data about prices, (and) what's popular," Cicilline said sceptically. "You're saying that you don't use that in any way to promote Amazon products? I remind you, sir, you're under oath."

Sutton argued that the data was used solely to predict what customers wanted, and to offer it to them. "The algorithms are optimised to what customers want to buy regardless of the seller," he said.

Matt Perault, head of global policy development at Facebook, faced scepticism from Representative Hank Johnson of Georgia when Perault stressed Facebook's many social media competitors. "I'd like to know who this competition is," Johnson said. "It's not readily apparent."

Lawmakers did not ask about antitrust probes of the four companies under way at the Justice Department and Federal Trade Commission.

Nor did they press Facebook about a proposed $5 billion settlement between with the FTC to resolve allegations that the company violated a 2011 consent agreement by inappropriately sharing information on 87 million users with the now-defunct British political consulting firm Cambridge Analytica.

Other congressional panels Tuesday focussed on Facebook's plans to bring out a cryptocurrency, the Libra, and allegations that Google is biased against conservatives in search results.

While the tech companies appear to have few friends on Capitol Hill, there has been some pushback from Republicans against a proposal by Senator Elizabeth Warren, who is running for president, that Amazon, Facebook and Google be forced to divest companies that they purchased previously.

Representative Kelly Armstrong and Representative Jim Sensenbrenner, both Republicans, cautioned the panel against going beyond the bounds of antitrust law.