US antitrust enforcers have asked a federal court for extra time in refiling a monopoly abuse case against Facebook which could roll back its acquisition of Instagram and WhatsApp, but was thrown out last month.

The Federal Trade Commission (FTC) said Facebook did not oppose the request to extend the filing deadline by some three weeks until August 19, according to a filing in court on Friday and posted online on Monday.

The extension gives the FTC time to revise its argument in the case.

Last month, Judge James Boasberg of the US District Court of Washington, DC, said in a 53-page opinion that the agency's initial lawsuit lacked evidence, notably in defining the market that Facebook was allegedly monopolising.

The federal agency based its case on a "vague" assertion that Facebook controlled more than 60 per cent of the social networking market, but the FTC "does not even allege what it is measuring", according to the judge's June 28 ruling.

The judge said the December FTC complaint "says almost nothing concrete on the key question of how much power Facebook actually had... it is almost as if the agency expects the court to simply nod to the conventional wisdom that Facebook is a monopolist".

The judge issued a separate opinion dismissing the case by the states, saying attorneys general had waited too long to bring the case for the acquisition of Instagram in 2012 and WhatsApp in 2014.

He initially gave the FTC 30 days to refile the case.

The new FTC filing said the agency will file an amended complaint by August 19, and that Facebook will have until October 4 to file its response. Additional briefs will come November 17 from the FTC and December 1 from Facebook.

The move comes amid a ramped-up effort by US antitrust enforcers against the largest technology firms which have increasingly dominated key sectors of the economy, including Facebook, Apple, Amazon and Google.