Donald Trump has prompted widespread criticism with a tweet suggesting the risk to Americans from coronavirus is "low".

The post comes amid a surge in the number of cases in the US to more than 500, while 22 people in the country are now dead and two US lawmakers from the president's own party are in self-isolation after coming into contact with a man who was diagnosed with Covid-19.

Mr Trump said in the post that the risk to Americans from the disease was being intentionally overstated, in what he appeared to suggest was an intentional attack on him despite providing no evidence to support that contention.

"The Fake News Media and their partner, the Democrat Party, is doing everything within its semi-considerable power (it used to be greater!) to inflame the CoronaVirus situation, far beyond what the facts would warrant," the president wrote. "Surgeon General, 'The risk is low to the average American.'"

The comments from the surgeon general were specifically related to the current risk - rather than being a reference to any long-term potential dangers the outbreak could bring - and indicated that people should nonetheless "prepare" for an increase in cases.

"What you're going to hear from the president is what you've heard from him all along: that the risk to the average American of coronavirus at this time remains low," Jerome Adams told Fox News on Friday.

"However, we are seeing pockets in this country of increased cases of coronavirus. And so, we want people to prepare."

Mr Trump's comments were immediately criticised, with experts pointing out that the tweet could serve to undermine efforts to fight the disease.

"This is dangerous. After a weekend of criticism of his administration's coronavirus response, in which he made promotional appearances at multiple Trump properties, the president is now denying the gravity of the situation," wrote watchdog group Citizens for Responsibility and Ethics in Washington.

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Others claimed the president appeared to be using the spread of coronavirus to attack his political opponents, rather than to safeguard the health of the public.

"It is [Donald Trump] who is politicizing this disease, not the Democrats," wrote MSNBC political analyst Richard Stengel.

"He is so deeply narcissistic that he sees a pandemic as a plot against him personally. Not the person who should be leading the fight against it. Catastrophically incompetent & cognitively unable to see the threat."

Brian Klaas, a political scientist at University College London, responded: "He’s almost certainly going to regret this tweet."