WASHINGTON - Money to fight the outbreak of the mosquito-borne Zika virus in the United States is running out, US President Barack Obama said on Thursday, calling the situation "critical" and urging lawmakers to act.

Obama spoke to reporters at the Pentagon, where he said the US government is taking the news of 15 cases of Zika being locally transmitted in Florida "extremely seriously".

"As our public health experts have been warning for some time, we are now seeing the first locally transmitted cases of the Zika virus by mosquitoes in the continental United States," Obama said.

"This was predicted and predictable."

Obama asked Congress in February to allocate US$1.9 billion for the fight against Zika, but was met with resistance by Republican lawmakers who said the funds should instead be moved from coffers previously reserved for fighting the Ebola outbreak.

"Not only did the Republican-led Congress not pass our request, they worked to cut it. Then they left for summer recess without passing any new funds for Zika," said Obama.

"Meanwhile, the people on the front lines have been making do. Now the money we need to fight Zika is running out."

Without funding, Obama warned, clinical vaccine trials could be delayed.

"The situation is getting critical," he said.

"So this is not the time for politics."

Zika symptoms are mild in most people and go unnoticed in four out of five cases.

However, the virus can cause severe birth defects, and pregnant women are being urged to stay away from the Miami area of Wynwood where mosquitoes with Zika are believed to be lurking.

Zika is currently spreading in more than 50 countries and territories of the world, particularly in Latin America. – AFP