Phase four is the highest level alert the WHO has issued since the system was put in place in 2005.
The WHO did not boost the level to an out-right pandemic, which is indicated by phase six, and it did not recommend closing borders or creating restrictions on travel because the virus is already seen on multiple continents.
"This virus is too widespread to make containment a feasible consideration," said Dr. Keiji Fukuda, WHO assistant director-general, recommending instead that nations focus on mitigation by providing citizens with necessary information to help prevent infection.
Fukuda said it is possible the alert would stay at a phase four for quite some time, but that if the situation worsens the WHO may need to change the warning level within days.
"Over the past few days we've seen how quickly the situation is evolving," he said.
Fukuda also said production of the seasonal flu vaccine should continue and that the WHO should facilitate creation of a vaccine for swine flue.
Governments around the globe are scrambling to respond to the threat of a swine flu outbreak that has killed as many as 103 people in Mexico, sickened dozens of people in the United States and Canada, and spread to Europe.
The first case of swine flu in Europe was confirmed Monday, as health officials in Spain said a man who returned from a trip to Mexico last week has tested positive for swine flu. Officials said 17 other suspected cases were being tested.
In response to the outbreak, the European Union's health commissioner issued a travel warning Monday urging Europeans to avoid trips to the United States or Mexico if possible.
Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention in Atlanta, Ga., told CNN the warning is unwarranted.
"We are looking very hard for cases of swine flu," he said Monday. "I expect we're going to find some, and we'll find some of increasing severity and more of the mild cases. At this point I would not put a travel restriction or recommendation against coming to the United States."
Listen to Besser's Monday CDC briefing on the swine flu situation:
The U.S. State Department plans to issue a travel warning on Monday for Americans to avoid all "nonessential" travel to Mexico because of the deadly outbreak there, a U.S. official told Reuters. Though broad travel restrictions have not been in place, stock prices for U.S. airlines tumbled Monday on concerns that swine flu will curb travel, reported Bloomberg.
The United States declared a public health emergency on Sunday to free up money to respond to a potential crisis. One-quarter of the national stockpile of 50 million antiflu drug regimens will be released as well, reported The New York Times.
But President Barack Obama said Monday that the swine flu outbreak is a "cause for concern" but not alarm and that the declaration was made as a precaution.
"The Department of Health and Human Services has declared a Public Health Emergency as a precautionary tool to ensure that we have the resources we need at our disposal to respond quickly and effectively," he said.
The case found in Spain, as well as the 20 cases identified in the United States and six in Canada have not resulted in serious illness or death. Only one of the cases in the United States has required hospitalization.
However, Besser told ABC News Monday that Americans should be prepared for the idea that "we could see more severe cases in this country and possibly deaths."
Other cases are suspected in New Zealand, Britain, France, Italy and Israel, reported Reuters.
Officials in Mexico, still the epicenter of the outbreak, encouraged people to stay home Monday if they feel they have symptoms.
The government closed schools nationwide until at least May 6 to help contain the disease. Most of the those who have died of swine flu were between 20 and 50 years of age, Health Minister Jose Angel Cordova said Monday.
The Mexico City government is considering a complete shutdown if deaths keep occurring, including public transportation.