McDonald's has apologised for its 'PSI' advertisement which drew flak when it was published in The Straits Times on Thursday.

The advertisement, which carried the sentence ""Today's Peak Sauce Index is looking deliciously high", was published the day after Singapore experienced its worst day of haze.

Said student King Rachel, 22, "If the advertisement was used when the PSI first came up, the joke might seem funny. But when the PSI hit 300, it's not a joke anymore. It has become a serious issue."

A spokesman from McDonald's said the ad was "not intended to trivialise the current haze situation". She added that the one-off ad was designed and placed when haze conditions were not as severe, and agreed that in this current context, the advertisement was "inappropriate".

"We are truly sorry that it has led people to feel that way," she said. "We share the concerns of Singaporeans and were equally surprised to see PSI levels hit record highs overnight."

As a precaution for the health and safety of its delivery riders, McDonald's halted its McDelivery service from Wednesday night.