Singapore - Singapore stepped up measures to guard against the swine flu outbread and elevated the flu alert level from green to yellow after the World Health Organization decided to raise pandemic alert levels from phase 3 to 4.

"This is to brace ourselves to handle situations where there may be isolated imported cases but no sustained local transmission," the Ministry of Health said in statement late Tuesday night.

So far there are no human cases of swine flu in Singapore.

However, 17 people, most of them Singaporeans who have recently returned from the United States, have been referred for further medical assessment, the ministry said.

So far, four cases had been tested negative for influenza A.

Preliminary test results would be available in a day but confirmation of the swine flu strain would take up to seven days.

As precautionary measure Singapore already deployed thermal scanners at the city state's airports to screen all arriving passengers.

Since Wednesday body heat scanners are also deployed at several international and regional ferry terminals. At other ferry piers thermometers are used to screen visitors, the ministry said.

It advised Singaporeans to postpone or avoid non-essential travel to Mexico.

Minister Mentor Lee Kuan Yew said that Singapore is taking no chances.

"We learnt from Sars. We've got all the precautions in space, thermal imaging; slightest sign of something wrong, you go straight into quarantine," the Straits Times newspaper quoted Lee as saying.

The ministry of health said it had ramped up isolation capacity in public hospitals since the outbreak of severe acute respiratory syndrome (SARS) in 2003 which killed 33 people in Singapore.

"We have stockpiled enough antiviral drugs like Tamiflu and Relenza to treat Singaporeans should the need arise," the ministry added.

It urged members of the public not to stockpile these drugs. "Indiscriminate use may ultimately limit their effectiveness as drug-resistant viruses may develop," the ministry said.

However, according to local media reports pharmacies around Singapore are running out of flu drugs as Singaporeans try to buy their own supplies.