Looters posing as firefighters and Homeland Security special agents are telling residents to evacuate in order to ransack their homes in a sinister twist to Hurricane Harvey.

An overnight curfew has been imposed in Houston after a spate of robberies and looting.

City officials announced the midnight to 5am clampdown after police arrested a crew of armed robbers who were hijacking vehicles and reports of thefts from some of the thousands of homes that lay partially underwater and abandoned by fleeing residents.

They warned people to ask anyone knocking on their doors for official badges and credentials with their name and organization.

San Leon volunteer firefighter Jody Garcia told the Houston Chronicle that one person posing as a first responder pointed a shotgun at two residents who were trapped in their homes and told them to leave.

US Immigrations and Customs Enforcement has confirmed they will not be conducting any operations in Houston during the rescue effort.

As Houston struggles with the devastation of Hurricane Harvey, lawmakers in Texas are vowing to be extra tough on anyone caught looting after authorities reported some minor thefts and armed robberies had taken place

'You cannot drive, nor be in any public place. We have had problems with armed robberies, with people with guns and firearms,' said Houston Police Chief Art Acevedo.

He said the Houston Police Department was going to have to stop assisting in search-and-rescue missions to focus on 'going after criminals and keeping the good people of Houston safe'.

The police chief said: 'This is the state of Texas. We're a welcoming city, but we are not going to tolerate people victimizing, especially committing armed robberies in our community.

'We're going to catch you, and I promise you this: We are going to push hard - I've talked to the district attorney - to seek the fullest prosecution possible available for any crime that is committed.'

Floodwaters in Houston are seen on Tuesday. Officials have vowed harsh penalties for looters

These Houston residents posted this picture on Monday vowing unpleasant consequences for anyone who loots during the flood or shoots at rescuers

An Apple store in Houston had its door blasted open with a gun before it was looted, according to reports.

Prosecutors issued a statement promising that looters would face tougher penalties under Texas law for committing burglaries during a crisis.

'People displaced or harmed in this storm are not going to be easy prey,' said Harris County District Attorney Kim Ogg. 'Anyone who tries to take advantage of this storm and break into homes or businesses should know that they are going to feel the full weight of the law,' she added.

'Offenders will be processed around the clock without delay.'

Mayor of Houston Sylvester Turner said: 'Just like we can rescue you we can also arrest you - obey the curfew, don't engage in looting, let's focus on rebuilding our city.'

'You loot, I shoot,' promised one spray-painted sign in a residential neighborhood of Portland, Texas, a small city right outside of Corpus Christi on the Gulf coast.

'If I needed to shoot somebody to protect my stuff if they broke into my house, I think that's appropriate,' one mother in the neighborhood told KIII, asking not to be identified.

'We're in a state of disaster,' she said.

'When you have people that have lost everything, you need food, you need clothes, people get desperate and you go into that kind of mindset,' the mother said.

The National Hurricane Center (NHC) said a record 51.88 inches (131.78 cm) of rain has fallen in Texas due to Harvey, a record for any storm in the continental United States

During Harvey relief efforts US Immigration and Customs Enforcement is not conducting immigration enforcement operations in the area.

In Houston people coming into shelters will not have their immigration status checked.

Authorities and family members have reported at least 18 deaths from Harvey, while law enforcement agencies said more than 13,000 people have been rescued in the Houston area and surrounding parts of Southeast Texas.

The storm that first came ashore on Friday has forced tens of thousands of people to flee deluged homes and caused damage estimated at tens of billions of dollars, making it one of the costliest US natural disasters.

The National Hurricane Center (NHC) said a record 51.88 inches (131.78 cm) of rain has fallen in Texas due to Harvey, a record for any storm in the continental United States.

As the floodwaters continued to rise, public health officials warned about the dangers of health problems ranging from skin rashes to bacterial and viral infections and mosquito-borne disease.

The fire marshal's office also tweeted that residents within 1.5 miles of a chemical plant in Crosby, Texas, were being evacuated as a 'precautionary measure' because of the rising risk of an explosion.

Neighboring Louisiana is set to bear the brunt of the tropical storm's massive downpours today, with flash flood warnings in place across the entire state.