The official injury toll after an explosion at a hotel in Fort Worth, Texas has risen to 21.

One person has been left in a critical condition, and four people were in serious condition, authorities said at a news conference on Monday.

Others had been taken to nearby hospitals for treatment of minor injuries.

The blast, which authorities believe may have been caused by a gas leak, blew out windows and saw entire sections of wall thrown out onto the road in front of the 20-storey Sandman Signature hotel.

A spokesman for the Fort Worth Fire Department, Craig Trojacek, said rescue crews found several people trapped in the basement, and more than two dozen rooms were occupied at the hotel when the blast took place.

“We had reports of people trapped down in the basement, and because of the explosion that took place, some of those access areas were either covered up or it didn’t feel safe at that point to get people down into,” Mr Trojacek said.

Mr Trojacek said a restaurant in the building had been under construction but said it was not definitive that is where the blast occurred.

“There is a smell of gas in the area and there are windows and things that were blown outside of the structure,” he said.

He said investigators were working with federal investigators to determine the cause of the blast. The scene had left rescue teams unable to immediately reach some parts of the building.

The Sandman Signature Hotel is in a busy area of downtown, about one block from the Fort Worth Convention Centre.

Footage from news helicopters showed firefighters picking their way through the piles of drywall, shattered glass and mangled metal that coated the street outside the hotel. Authorities urged people to avoid the area.

People in buildings nearby the hotel on Monday afternoon recalled hearing a loud crack that sounded like thunder and then seeing a wall of dust and papers sweep through the city streets.

“There was debris. There was insulation. There was office furniture,” Charlie Collier, 31, told The Associated Press.

“Everything that was in the first couple floors of the building was blown out all over the street.”

Rebecca Martinez, who works across the street from the hotel, said she stepped outside and came upon a man and a woman leaning up against a fire hydrant.

“The man was all bloody. His face was all bloody,” she said. “Then I started smelling natural gas, real intense.

“I thought, ‘I might need to get away from here’.”

Moments later, authorities evacuated her building and some of the surrounding neighbourhood.

A grey haze covered the normally busy streets of downtown Fort Worth as firefighters walked through layers of debris. Remnants of the building lay scattered across the street and over parked vehicles, and gaping holes could be seen on the ground.

Texas state representative Nicole Collier, whose district office is about eight blocks from the hotel, said she did not hear the explosion but heard the sirens of responding emergency vehicles and helicopters overhead.

Ms Collier said she tried walking towards the site, but emergency responders had already closed several blocks and were warning people to stay away.

According to the hotel website, the Sandman Signature Fort Worth Downtown Hotel has 245 rooms and was built in 1920 as the Waggoner Building, named after cattle rancher and oilman William Thomas Waggoner.

The building has been listed on the National Register of Historic Places since 1979.