The World Health Organization isn't yet ready to advise vaccine makers on whether they should divert production to make swine flu shots instead of vaccines for seasonal flu, it said Tuesday.

"We are in the process of the most basic development of the vaccine," acting WHO assistant director general Keiji Fukuda told reporters.

Research on candidate swine flu vaccines will continue until the end of June or July, at which time drug companies could start to test a possible vaccine for H1N1 influenza, he said. But without knowing how effective the vaccine might be, it's too early to recommend that drug makers commit manufacturing resources to it, Fukuda said.

The WHO also said it would consult influenza experts before formally changing its criteria on when to declare a pandemic. The UN health agency has faced pressure from several countries to consider the mild severity of swine-flu infections so far as a deterrent to raising its pandemic alert to Phase 6, the highest. Normally, the alert scale doesn't consider the severity of an illness itself, but the WHO said it was amenable to changing that, if experts agreed.

Earlier this week, it was confirmed that the U.S. government has awarded $660 million US in contracts to three pharmaceutical giants to make swine-flu vaccines.

The companies — GlaxoSmithKline, Sanofi Pasteur, and Novartis — are to receive seed virus from the U.S. Centers for Disease Control and Prevention, with the aim of starting commercial production in June.

The virus has been linked to 92 deaths and infected 12,954 people in 46 countries since it was first uncovered last month, the WHO said in its update Tuesday.

The Persian Gulf kingdom of Bahrain reported its first confirmed case of H1N1 swine flu on Monday.

In the U.S., New York City's health commissioner announced Tuesday that two more deaths have been linked to swine flu, bringing the number of deaths in the city to four.

In its most recent update on Monday, the Centers for Disease Control said there were 6,764 confirmed cases of swine flu in the U.S., most of them mild.

The Public Health Agency of Canada's most recent update on Monday reported 921 confirmed cases in nine provinces and one territory.

With files from The Associated Press