Countries with large numbers of cases of a virulent pneumonia should screen passengers at airports for the illness, say experts.
A mother and daughter wear masks
The World Health Organisation says that air passengers leaving Hong Kong, Taiwan, Hanoi, Singapore, Toronto and Guangdong province in China should be asked a series of questions to try to spot those carrying the SARS virus.
The illness looks likely to have killed more than 50 people worldwide, and air travellers have carried it to more than a dozen different countries in the space of a week.
A WHO spokesman said that the illness seemed to be spread by "close contact", meaning an infected passenger could spread the disease to anyone sitting within a few rows of seats on the plane.
Spokesman Max Hardiman, WHO project leader for international health
regulations said: "We are going to step up our travel recommendations."
The US Centers for Disease Control said on Thursday that a Corona virus - possibly a strain never before seen in humans - is to blame.
Other types of Corona virus can cause the common cold.
Any airline has the right to refuse to carry someone they have good reason to believe is "unfit to fly".
However, the WHO has also given instructions to staff on what to do if they spot someone on board a flight who has symptoms.
Experts say they should try to "isolate" him or her from fellow passengers, and encourage the patient to wear a face mask.
Increasing concern over the virus has prompted the closure of schools in both Hong Kong and Singapore until April 6.
The Hong Kong government has ordered people who believe they have been exposed to the virus to stay away from work - or be fined or jailed.
Have you been in contact with anyone who has SARS?
Have you had symptoms such as a dry cough or high fever?
Has any member of your family had any contact with someone with SARS?
The Chinese government revealed on Wednesday that 34 people had so far died in an outbreak of a similar respiratory illness - mostly in the southern province of Guangdong.
The World Health Organisation says that it believes the same virus is responsible for both the Guangdong outbreak and the "SARS" illness which has killed 19 and infected hundreds more elsewhere in the world.
Dr Malik Peiris from Hong Kong University told a news conference: "It appears to us that the Corona virus is the primary cause of the disease...it is possible of course that other viruses might also infect the same patient and might increase the severity of the disease."
He suggested that the SARS strain might have originated from an animal virus which mutated to infect humans.
SARS so far - cases (deaths)
Hong Kong - 357 (11)
Singapore - 74 (2)
Vietnam - 59 (4)
Canada - 33 (3)
Britain - 3 (0)
US - 40 (0)
China - 792 (34)
The south China region has historically been associated with animal viruses "jumping" to humans.
The identification of the virus could allow doctors to develop a test to rapidly identify genuine SARS sufferers.
All Hong Kong schools have been ordered to shut until at least April 6 to reduce the spread of the disease, it was announced on Thursday.
More than 350 people in Hong Kong are now known to have been infected, and 11 have died.
The authorities there have imposed a quarantine law.
People who think they may have come into contact with a person known to be infected have to report for assessment at a clinic - and must stay away from work for 10 days.
Singapore has reported its first death from the illness, and has also announced the closure of all schools until the same date.
Likely culprit: Coronavirus
Almost 500 people outside China have been infected with the illness, according to most recent World Health Organisation figures.
New suspected cases have also emerged in Canada, where three people have already died from SARS.
In Singapore, the Ministry of Health has invoked special powers to keep 740 people under home quarantine in an attempt to try to halt further spread of the disease.
Bookings on Singapore Airlines have plummeted since the first cases emerged, tourism authorities