CHICAGO (AFP) - A large plume of black smoke hovered over parts of the US city of Houston on Tuesday (March 19) as firefighters struggled with a days-long chemical plant fire.

The blaze broke out Sunday at an Intercontinental Terminals Company (ITC) plant - sending orange flames into the sky and thick black smoke wafting over the Texas city.

The plant in the suburb of Deer Park makes chemicals used in gasoline mixtures and paint thinners. The fire spread overnight from six to eight storage tanks, according to ITC.

The chemicals could cause coughing, difficulty breathing, irritation to the eyes, nose and throat, headaches, nausea or dizziness, according to the Harris County Public Health Department.

Officials closed nearby schools on Monday, but reopened them on Tuesday after tests reassured officials of the surrounding air quality.

"Air monitoring continues and as of this update readings are currently well below hazardous levels," ITC said in a statement on Tuesday.

"There are no injuries reported at this time."

But sceptical residents pointed to the plume of smoke spewing from the plant two days after the fire started. Many parents wrote on social media that they would not send their children back to school.

"My son's school is a couple of miles away. I will not gamble with his life over these false reports," one parent wrote on the Deer Park school district's Facebook page.

The Deer Park school district said it has received "several emails and calls from parents regarding the decision to resume school."

Officials said it could take until Wednesday for the fire to be extinguished.