The White House has rebuked the top US health agency, saying "it let the country down" on providing testing crucial to the battle against the coronavirus outbreak.

The Centers for Disease Control and Prevention has been under intense scrutiny since producing a faulty test for Covid-19 that caused weeks of delays in the US response.

Critics have pointed out that it could simply have accepted kits made by the World Health Organization, which has been producing them since late January, instead of insisting on developing its own test.

"Early on in this crisis, the CDC, which really had the most trusted brand around the world in this space, really let the country down with the testing," White House trade adviser Peter Navarro told NBC's "Meet the Press."

"Because not only did they keep the testing within the bureaucracy, they had a bad test. And that did set us back."

The Food and Drug Administration has also criticised the CDC for not following its own protocols in manufacturing Covid-19 tests. The errors were not corrected until late February.

US President Donald Trump often blames the administration of his predecessor, Barack Obama, for passing on "broken tests" for the new coronavirus - although Mr Obama left office years before the virus came into existence. 

However Mr Navarro's comments mark the strongest criticism by a named White House official of the CDC's role in the administration's slow roll out of testing.

Health and Human Services Secretary Alex Azar defended the CDC against Mr Navarro's criticism, telling CBS it was never meant to be "the backbone of testing, of broad, mass testing, in the United States."

"I don't believe the CDC let this country down. I believe the CDC serves an important public health role. And what was always critical was to get the private sector to the table," he said on "Face the Nation." 

An editorial in the respected medical journal The Lancet also came to the agency's defense.

"There is no doubt that the CDC has made mistakes, especially on testing in the early stages of the pandemic... But punishing the agency by marginalising and hobbling it is not the solution," it said. 

"A strong CDC is needed to respond to public health threats, both domestic and international."

The United States has the world's largest coronavirus outbreak by far, with more than 89,000 deaths among nearly 1.5 million confirmed cases, according to Johns Hopkins University. 

On 6 March, Mr Trump said as he toured the CDC headquarters in Atlanta that four million "beautiful" testing kits would be available within a week and that "anybody that needs a test gets a test." 

More than two months later, just 12 million Americans have been tested - less than 4% of the population. 

That places it 39th in the world behind other hard-hit countries like Russia, Italy and Spain, according to trusted online statistics source Worldometer.

Experts say widespread testing - of healthy people as well as those with symptoms - is crucial as a means of knowing exactly where the virus is spreading as the US begins reopening the world's biggest economy.  

In New York, the hardest-hit part of the US, Governor Andrew Cuomo took a virus test on live television in a bid to encourage more widespread testing and pave the way for a safer reopening of the populous state.

Mr Navarro, echoing a claim made frequently by the president, said that locking down the country until the outbreak was over would kill "a lot more people" through suicides, drug abuse and a halt on medical procedures than the virus itself. 

He accused China - where the outbreak started late last year - of crippling the US economy "in 30 days," but vowed that Mr Trump would rebuild it as the country reopened.