Exaggerated fears of Sars are having an unnecessarily damaging economic impact, says the World Health Organization.
China reported eight more deaths on Monday
The pneumonia-like disease is hitting tourism and trade "due in part to a discrepancy between the real and the perceived threat," WHO head of communicable diseases Dr David Heymann told a Bangkok news conference.
The comments came hours after Dr Heymann said the spread of Sars had peaked in most countries identified as having the disease since 15 March - except in China, the world's worst-hit area, and Taiwan.
China's health ministry reported eight new Sars deaths and 203 new cases on Monday, bringing the nationwide death toll to 139 and total cases to more than 3,000.
This disease does not spread easily; there is no need to wear masks on the main street.
WHO head of communicable diseases Dr David Heymann
But Vietnam - one of the first countries to be hit - was declared by the WHO to be the first country to have brought the disease under control, after reporting no new cases since 8 April.
Dr Heymann was quoted by Reuters news agency as saying that fears of catching Sars should not prevent people travelling on aircraft as there was solid evidence of only five incidences of onboard Sars transmission.
"This disease does not spread easily; there is no need to wear masks on the main street. There is no evidence that this is aerosolised throughout an aircraft," he said.
But the WHO is continuing to advise against travel to Beijing, Hong Kong, China's Shanxi and Guangdong provinces and the Canadian city of Toronto, which have all been hard-hit by the Sars virus.
'China is the key'
Dr Heymann, asked if he was confident that the worldwide spread of Sars could be stopped, said: "No we are not. We are hoping.
"China is the key and it's the unknown question in the whole formula, because if China cannot contain it, then it can't be removed."
The Chinese authorities are continuing to step up measures to contain the virus.
KNOWN DEATH TOLL
Mainland China: 139
Hong Kong: 138
Indonesia: 1 (unconfirmed)
Source: WHO/ local health authorities
An estimated 8,000 people are now under quarantine in Beijing, and the authorities have shut down all cinemas, theatres, internet cafes and schools.
BBC correspondent Rupert Wingfield-Hayes says the capital is being turned into a ghost town, with streets and offices virtually deserted as people stock up and shut themselves in their homes.
The leadership hopes the sense of crisis felt by ordinary Chinese will prevail over their anger at the government's failure to respond quickly and transparently to Sars, says the BBC's Francis Marcus in Shanghai.
But some critics of the Communist Party think Beijing's belated openness could further undermine the credibility of China's new government, our correspondent adds.
Dr Heymann said the WHO did not recommend closing theatres and night clubs, adding: "As China discovers more about Sars, I'm sure they'll reconsider their decision."
The number of Sars deaths worldwide is now well over 300, and the number of cases about 5,000.
The Swiss pharmaceutical company Roche has said it is hoping to launch a diagnostic test for the virus by the end of July.
Hong Kong's government said on Monday that a further five people had died from the disease - bringing its death toll to 138 - and there were reports of the first Sars death in Indonesia.
The WHO said it had taken Vietnam off its list of countries with "local transmission" of Sars - but the country is taking additional measures to prevent the spread of the disease by travellers.
But Vietnam is reportedly considering a temporary closure of its land border with China.
Leaders of the Association of Southeast Asian Nations (Asean) are meeting in Bangkok on Tuesday for a half-day emergency summit.
China's Premier Wen Jiabao will also attend the talks, which are being seen as a crucial step towards adopting a common stance against the spread of Sars.
Several Asian countries are already introducing strict new measures.
Taiwan has imposed a two-week ban on visitors from badly affected areas, after the island announced its first Sars death on Sunday.
The South Korean Government said it would increase airport checks and free up hospital space, despite the fact it has still not had any confirmed cases of the virus.
In the Philippines - which has recorded four cases and two deaths - the government said violators of strict new quarantine laws could be detained by police.
An estimated 200 people in one town, where a nursing assistant died, have been placed under mass quarantine.
Vietnam has heightened controls along its long China border
Cases in Singapore appear to be declining, with the imposition of stringent measures including thermal imaging of air passengers to detect those with high temperatures.
Toronto - the only place outside Asia where people have died from the illness - is to host an international conference on the virus.
Officials are hoping the WHO will change its current advice for people to avoid visiting the city - they say the general public has nothing to fear.