Toxic smoke billows from a gasoline storage and distribution centre that continued to burn Saturday following a massive explosion on Friday morning. (Puerto Rico police/Associated Press)

Thousands of people living downwind from a burning fuel-storage site in Puerto Rico were urged to leave on Saturday to avoid toxic smoke still billowing from the blaze.

More than 1,500 people were forced to flee after Friday's earth-shaking explosion at the Caribbean Petroleum Corp. in suburban Bayamon, just west of the capital, San Juan.

Only a few people were slightly injured by the blast, which broke windows, tore doors off their hinges and shook the ground.

Authorities are now concerned about those downwind of the fire, with breezes pushing smoke toward more populated neighbourhoods. They're urging residents, especially those with respiratory problems, to temporarily move elsewhere until the fire is extinguished and the smoke is brought under control.

Several people have sought medical help for respiratory distress and one firefighter was treated Saturday for exposure to high temperatures, Gov. Luis Fortuno said.

Fortuno said a large stadium has been prepared to accommodate some 30,000 people who live in the area if necessary.

Adding to the danger posed by toxic, sulphurous smoke is the presence of invisible gases such as carbon monoxide, said Luis Antonio Ocasio, spokesman for Puerto Rico's Environmental Quality Board.

Local officials along with a crew from the U.S. Environmental Protection Agency are monitoring the air with hand-held devices and checking whether any fuel spilled into the nearby ocean, he said.

The cause of the explosion is still under investigation. Fifteen of 40 fuel tanks caught fire, and five were still burning Saturday afternoon.

With files from The Associated Press