Washington DC - A US House subcommittee concluded a two-year investigation Monday into the Covid-19 outbreak that killed 1.1 million Americans, backing the theory that the virus likely leaked from a Chinese laboratory.

A 520-page, highly politicized report from the Republican-controlled House Select Subcommittee on the Coronavirus Pandemic looked at the federal and state-level response, as well as the pandemic's origins and vaccination efforts.

"This work will help the United States, and the world, predict the next pandemic, prepare for the next pandemic, protect ourselves from the next pandemic, and hopefully prevent the next pandemic," panel chairman Brad Wenstrup said in a letter to Congress.

US federal agencies, the World Health Organization, and scientists across the planet have arrived at different conclusions about the most likely origin of Covid-19.

Most believe it to have spread from animals, but a US intelligence analysis said last year that the virus may have been genetically engineered and escaped from a virology lab in the Chinese city of Wuhan, where human cases first emerged.

The congressional panel went for the lab leak theory after meeting 25 times, conducting more than 30 transcribed interviews, and reviewing more than one million pages of documents.

The investigation included two days of interviews behind closed doors with Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases who became the leading US expert in the chaotic early days of the 2020 outbreak.

Fauci's clashes with former and incoming president Donald Trump over the response sparked fury on the right, and he now lives with security protection following death threats against his family.

Republicans accuse the 83-year-old scientist of helping to set off the worst pandemic in a century by approving funding passed on to Chinese scientists they allege manufactured the SARS-CoV-2 coronavirus that causes Covid-19.

Among its headline conclusions, the report said the National Institutes of Health had indeed funded contentious "gain-of-function" research – which seeks to enhance viruses as a way of finding ways to combat them – at the Wuhan Institute of Virology.

Fauci angrily denied covering up the origins of Covid-19 before the panel in June, arguing that it would be "molecularly impossible" for the bat viruses studied at the lab to be turned into the virus that caused the pandemic.

The House report contains other extremely contentious claims, such as accusing the World Health Organization of kowtowing to the Chinese Communist Party.

It also claimed that lockdowns "did more harm than good" and that mask mandates were "ineffective at controlling the spread of Covid-19," contradicting ample research showing that masking in public does reduce transmission rates.

There was, however, praise for travel restriction and the rollout of the vaccine program under Trump.

Beijing hit back at the report on Tuesday, saying it had "no credibility" and accusing the US of using the outbreak for "political manipulation."

"The authoritative scientific conclusion drawn by the China-WHO joint expert team... is that a laboratory leak is extremely unlikely," foreign ministry spokesperson Lin Jian told a regular news conference.

"In the absence of any substantive evidence, the so-called US report has concocted leading conclusions, slandered China (and) planted false evidence," he said.