The World Health Organization rang a global alarm over the Zika virus on Thursday, saying that the disease was “spreading explosively” in the Americas and that as many as four million people could be infected by the end of the year.

The global health agency will convene a special meeting on Monday to decide whether to declare a public health emergency. It is moving swiftly to combat this outbreak after widespread criticism that it had allowed the last major global health crisis, Ebola, to fester without a coordinated, effective strategy.

Since last spring, more than 20 countries have reported locally acquired cases of Zika, which is transmitted by mosquitoes and may cause birth defects.

“The level of alarm is extremely high,” said Dr. Margaret Chan, the director general of the W.H.O., in a speech in Geneva.

The focus of concern is the growing number of cases of microcephaly, a rare condition in which infants are born with abnormally small heads and damaged brains. Reports of babies born with microcephaly have been rising sharply in Brazil as Zika spreads.

Experts say it is too early to tell whether Zika is the cause of the condition, but there are indications that the two are linked. The government of El Salvador has gone so far as to advise women to refrain from becoming pregnant until 2018.

Even as international health authorities sounded strong warnings, health officials in the United States sought to reassure Americans, saying that the vast majority of those exposed to the virus never have symptoms and that the risk of a homegrown outbreak is very low, largely because of more effective mosquito control.

“For the average American who is not traveling to this area, there is nothing they need to worry about,” said Dr. Anne Schuchat, principal deputy director of the federal Centers for Disease Control and Prevention.

Still, health officials in the region and internationally say it is urgent to stop the virus now. Zika’s rapid rise and the specter of associated birth defects have major repercussions in a warm region with struggling economies and vast flows of tourists, all ingredients for further spread of the virus. Brazil is preparing to host the Olympics this summer, and Zika has cast a long shadow over those plans. The C.D.C. has advised pregnant Americans to avoid travel to the region.

Dr. Thomas R. Frieden, the director of the disease centers, briefed President Obama this week, and a team of experts from the C.D.C. is in Brazil working with the authorities. The Pan American Health Organization, or PAHO, the W.H.O.’s regional office responsible for the affected area, has been tracking cases since May. Some experts complained that the agency’s head office in Geneva had been slow to act, echoing the agency’s tepid reaction at the outset of the Ebola epidemic in 2014. Officials sought to dispel that impression.

“The aim here is to ensure that we are ahead of the curve,” Marcos Espinal, director of the Department of Communicable Diseases and Health Analysis at PAHO, said in an interview this week.

Global health authorities face the delicate task of alerting the world to the dangers of Zika without provoking panic. They are trying to mobilize a forceful response in an effort to avoid the failures of flawed intervention that hobbled the fight against Ebola in West Africa. Monday’s meeting could clear the way for a much more aggressive response.

But the diseases are very different. Ebola was incredibly deadly, and it spread through contact with bodily fluids. Zika is not known to be fatal, and it has mild symptoms. With inconclusive evidence that the virus is the cause of the birth defects and other ailments like a temporary form of paralysis called Guillain-Barré syndrome, health officials are cautious about drawing too dire a picture.

That has provoked a certain hesitation about what tone to strike, even within the W.H.O. Dr. Bruce Aylward, a W.H.O. assistant director general, said the situation was not alarming, the very word Dr. Chan, the head of the agency, used earlier in the day

“ ‘Concerned’ is certainly the right language to be used,” he said. “ ‘Alarmed’ would definitely not be the right language.”

The biggest unresolved question is whether infection by the Zika virus can cause microcephaly. “Based on the information so far, there is more and more concern that there may be a causal relationship, but a lot of the work so far is to rule out other possible causes,” Dr. Aylward said.

Numbers released by Brazil on Wednesday heightened the confusion. The Health Ministry said it had examined over 700 reported cases of microcephaly and found Zika in only six of the infants, though what that means exactly is unclear. Infectious-disease specialists caution that Brazil’s testing methods are outdated and may miss many Zika cases. They also say that in some cases, the mother may have had Zika, causing microcephaly in her baby, even if the virus is never detected in the infant.

“The level of concern is high, as is the level of uncertainty,” Dr. Chan said. “Questions abound. We need to get some answers quickly.”

“As long as we are on the ball, if somebody is sick they get removed from proximity to mosquitoes,” said Dina M. Fonseca, an entomology professor at Rutgers University-New Brunswick.

She added that her state, New Jersey, has a plan in place for chikungunya, another related virus, that includes testing mosquito pools, and if any are positive, taking action, which could include spraying. If someone is infected locally — not during travel to another country — that also prompts mosquito control, she said. She expected similar plans to be laid for Zika by summer.

The virus was first identified in 1947 in Uganda, but for decades it afflicted mainly monkeys. In 2007, the first outbreak was documented in the Pacific islands, in the Federated States of Micronesia. Then, in May 2015, Brazil reported its first case of Zika virus disease. Since then, the disease has spread within Brazil and to more than 20 other countries in the region, the W.H.O. said.

Dr. Espinal said the W.H.O.’s regional offices began working with the countries on the problem last May and set up an incident control center. Dr. Aylward said the health agency held a meeting to address microcephaly in November, after discovering an increase in cases in one part of Brazil. But experts have still not nailed down the link. The C.D.C. said it planned to send a team of epidemiologists to Brazil next month to work on the question.

One worry is that there is no vaccine against the virus or a rapid diagnostic test to determine whether someone has been infected. Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said in an interview this week that scientists were working on both.

“We are already on our way on the first steps to developing a vaccine,” he said. “And we have started to work on a diagnostic to tell if someone’s been infected.”

But Dr. Fauci said at a news conference that it was unlikely a vaccine would be widely available this year, or even in the next few years.

Many Americans have roots in Latin America, and with the rapid pace of modern travel, there will be many people who come to the United States with the virus. At least 31 cases of the virus have been reported in 11 states and the District of Columbia, but all of those patients were infected in other countries, Dr. Schuchat of the C.D.C. said. Additionally, New York State reported several cases Thursday.

But that is different from a broad public health epidemic, which experts said was unlikely. The United States, like other developed countries, is good at breaking transmission of the virus. Windows have screens, and homes have air-conditioning, keeping infected mosquitoes away from people. They also keep mosquitoes from contracting the virus from infected people and spreading it further. The small clusters of infections from a related virus — dengue — that have popped up in recent years in the continental United States did not spread.