Two explosions were reported at a flooded Texas chemical plant near storm-battered Houston Thursday, just as the region began its slow recovery following Harvey's onslaught.

Operators at the Arkema Inc facility said the Harris County Emergency Operations Center notified them at approximately 2:00am CDT (0700 GMT), of "two explosions and black smoke" rising from the plant in Crosby, a town about 25 miles northeast of Houston.

Arkema warned that "a threat of additional explosion remains," and urged people to stay out of the evacuation zone.

The Harris County Fire Marshal's Office in a later tweet confirmed "a series of chemical reactions at the @Arkema_Inc Crosby facility," which manufactures organic peroxides that can combust if not cooled to proper temperatures.

The incident confirmed the fears of operators who had already ordered the evacuation of an area within 1.5 miles (three kilometers) of the plant, which operators had said was at risk of exploding due to a "critical issue" triggered by monster storm Harvey's torrential rains.

The Harris County Sheriff's Office said one deputy had been taken to the hospital after inhaling fumes from the plant, with nine others driving themselves there "as precaution."

In a later tweet the sheriff's office said company officials said "the smoke inhaled by 10 deputies near plant in Crosby is beloved to be a non-toxic irritant."

But Arkema also said in a statement that "exposure to organic peroxides may cause eye, skin and/or respiratory irritation. The smoke may also contain organic peroxide degradation products, including hydrocarbons and alcohols."

The company said those products could cause eye, skin or respiratory irritation as well as nausea, drowsiness or dizziness, and urged residents within the 1.5-mile evacuation area to turn off their air conditioners to avoid potential smoke exposure.

Pence visits

East of the explosion site rural areas of Texas were drenched as Harvey headed eastwards, with the city of Port Arthur especially hard hit.

Vice President Mike Pence was to visit Texas Thursday to meet victims of the torrential rains and assess damage.

Louisiana authorities scrambled to safeguard their state from Harvey, whose onslaught evoked painful memories of Hurricane Katrina's deadly strike 12 years ago -- but New Orleans escaped with minimal rain.

Officials believe at least 33 people have been killed in the storm, and expect the toll to rise further -- although many of those missing may simply have no phone or access to power.

More than 30,000 people have found refuge in shelters across the Lone Star State, from the giant Houston convention center to small churches, according to the head of the Federal Emergency Management Agency (FEMA), Brock Long.

In Houston, where Mayor Sylvester Turner issued an nighttime curfew to aid search efforts and thwart potential looting, the two major airports had reopened on a limited basis, signaling a slow return to normality.

The National Hurricane Center downgraded Harvey to a tropical depression late Wednesday, but warned life-threatening flood conditions remain in southeast Texas and southwest Louisiana.

'A few pops'

At least a quarter of Harris County, which includes Houston and Crosby, is still under water.

Crosby resident John Villarreal, 45, told AFP he learned of the Arkema incident after leaving his home -- situated about a mile from the facility -- to survey flooding in the neighborhood when he saw "a lot of smoke, and you could see the flames in the smoke."

"We could hear a few pops," he said. "I would call it like an aerosol can in a fire type deal."

Villarreal -- who spent five years working at the plant making organic peroxide approximately two decades ago -- said he and many neighbors did not evacuate the area because "there was really no clear direction" from authorities concerning potential risks of staying.

He is currently sheltering 10 family members and neighbors whose homes were flooded as Harvey for days pummeled the region with record rainfall.

Despite the explosion risks Villarreal said he had wanted to stay in Crosby to assist elderly neighbors in the event of emergency.

"We're all invested heavily in this area so we're doing the best we can to not let the worst happen," he said.

Life post-Harvey

Even as Houston got a first glimpse of life after Harvey, the city of Port Arthur was pummeled by torrential rains -- making it almost impossible to reach despite the best efforts of volunteer rescuers.

So far, parts of Texas have seen more than 50 inches (1.27 meters) of rain, while in Louisiana, the top total 18 inches so far was increasing.

Harvey first slammed onshore as a Category Four hurricane Friday night before unloading on Texas.

As a tropical storm it then made its second landfall just west of the Louisiana town of Cameron early Wednesday with maximum sustained winds nearing 45 miles per hour.

Heavy downpours were expected to stop on Thursday, though "life-threatening" flooding was to continue in southeastern Texas and southwestern Louisiana.

While New Orleans appeared to have dodged a bullet, Harvey's arrival served as a somber reminder -- coming just one day after the 12-year anniversary of Katrina, which ravaged the vulnerable city famous for its jazz music and cuisine.

In Texas, the storm damage is staggering -- Enki Research put its "best estimate" at between $48 billion and $75 billion.

At least one bridge crumbled, one levee had breached, and dams were at risk. Harvey also forced several major refineries across what is a major US oil industry hub to close.

More than 4,500 people and 1,000 pets have been rescued from the storm zone by the Coast Guard, which expected to rescue an additional 1,000 people Wednesday in the Port Arthur area alone.