The Zika virus is “spreading explosively” in the Americas, which could see up to 4 million cases over the next year, international health officials said Thursday, announcing a special meeting next week to decide if they should declare an international health emergency.

The warning from the World Health Organization came amid a call to arms by officials on both sides of the Atlantic over the mosquito-borne virus, which has been linked to a spike in a rare birth defect in Brazil.

“As long as we don’t have a vaccine against Zika virus, the war must be focused on exterminating the mosquito’s breeding areas,” Brazilian President Dilma Rousseff said.

The U.N. health agency called the special session in part to convey its concern about an illness that has sown fear among many would-be mothers.

Meanwhile, U.S. health officials said Thursday while they have not yet seen spread of the disease in the 50 states, the number of U.S. travelers infected over the last year in the Caribbean or Latin America has climbed to 31.

The Zika virus was first discovered in Africa in 1947. But until last year, when it was found in Brazil, it had never been a threat in the Western Hemisphere.

The virus causes no more than a mild illness in most people. But there is mounting evidence from Brazil suggesting infection in pregnant women is linked to abnormally small heads in their babies — a birth defect called microcephaly.

Earlier this month, U.S. health officials advised pregnant women to postpone visits to Brazil and other countries in the region with outbreaks.

But “for people who are pregnant and considering travel to the affected areas, please take this seriously,” Anne Schuchat, of the U.S. Centers for Disease Control and Prevention, said. “It’s very important for you to understand that we don’t know as much as we want to know about this yet.”