A new drug-resistant "super-flu" may pose a serious threat to high-risk patients, scientists say.

The H1N1 "A" virus is a strain that causes common seasonal flu outbreaks. But it carries an altered gene that makes it resistant to the anti-viral drug oseltamivir.

Marketed as Tamiflu, the drug is one of the first lines of defence used to protect people most vulnerable to flu.

Doctors today reported on the emergence of Tamiflu-resistant flu in the Netherlands and US.

They fear it could seriously threaten hospital patients with weakened immune systems.

Two separate teams of researchers highlighted the danger in the Journal of the American Medical Association (Jama).

The Dutch group led by Dr Jairo Gooskens, from Leiden University Medical Centre, said the rapid spread of resistant H1N1 flu strains had been observed since January 2008.

Viruses with the gene mutation, labelled H274Y, were originally thought to be less virulent and less easily transmitted.

The Dutch researchers wrote: "However, current widespread circulation of oseltamivir-resistant influenza A (H1N1) viruses associated with typical influenza illnesses and viral pneumonia suggest that these viruses retain significant transmissibility and pathogenicity."

Dr Gooskens' team identified four hospital patients with the resistant flu virus, and found they were most likely to have been infected in hospital.

Two patients were stem cell recipients and another was elderly.

Three of those affected developed pneumonia and two died, all of whom had weakened immune systems. Five health care workers also developed an influenza-like illness, but it was not possible to confirm whether they had picked up the virus.

The authors wrote: "The study confirmed that circulating H274Y-mutated A (H1N1) viruses can retain significant pathogenicity and lethality, as shown in these elderly or immunocompromised patients.. underling the urgency for the introduction of new and effective antiviral agents and therapeutic strategies."

The US team led by Dr Nila Dharan, from the Centres for Disease Control and Prevention in Atlanta, Georgia, found that 12% of influenza A (H1N1) viruses tested during the 2007-2008 flu season were resistant to Tamiflu.

But preliminary findings from the 2008-2009 season suggested much higher levels of Tamiflu resistance. As of February 19 this year, resistant strains had been identified among 264 out of 268 - or 98.5% - of influenza A (H1N1) viruses tested in the US.

The researchers examined data on 99 people infected with Tamiflu-resistant flu during the 2007-2008 influenza season. Five patients were hospitalised and four died.

"The emergency of oseltamivir resistance has highlighted the need for the development of new antiviral drugs and rapid diagnostic tests that determine viral subtype or resistance, as well as improved representativeness and timeliness of national influenza surveillance for antiviral resistance," the scientists wrote.