The mosquitos that carry the virus could reach more US states than previously thought, warned Dr Anne Schuchat of the US Centers for Disease Control and Prevention.

She also warned that a wider range of birth defects has been linked to the virus.

The Zika outbreak began almost a year ago in Brazil, and has been linked to thousands of birth defects there. It has also spread widely through the Americas.

At a White House briefing on Monday, Dr Schuchat said:  "Most of what we’ve learned is not reassuring.

"Everything we know about this virus seems to be scarier than we initially thought."

Earlier this year, US president Obama requested $1.8 billion from US Congress in emergency funding to combat the virus.

It has been using money totalling $589 million left over from the Ebola virus fund. However Dr Anthony Fauci of the National Institutes of Health warned that this was a temporary stopgap, and was inadequate to get the job done.

He added that the US now needed more money to fight the mosquitoes and to fund better research into vaccines and treatments, the BBC said.

"When the president asked for $1.9 billion, we needed $1.9 billion."

He said there had also been recent discoveries about how destructive Zika appeared to be to foetal brains, while there were also reports of rare neurologic problems in adults.

The first US case of locally transmitted Zika was reported in Dallas, Texas in February - spread through sexual contact, rather than a mosquito bite.