The death toll from Hurricane Harvey has risen to 15, as a family of six are believed to have been killed as they tried to escape unprecedented flooding in the Texan city of Houston.

Relatives said four children and their two great-grandparents are presumed dead after the van they were trying to flee in was swept into a river and sank.

Samuel Saldivar, the children’s great-uncle who was driving the van, managed to escape but Manuel Saldivar, 84, his wife Belia, 81, Devy Saldivar, 16, Dominic Saldivar, 14, Xavier Saldivar, 8, Daisy Saldivar, 6, are all believed to have died.

The driver’s sister-in-law, Virginia Saldivar, said Samuel had phoned her husband to tell them the van had been swept away by the flood waters while crossing a bridge.

"Sam calls my husband and tells him: 'They're gone’," she said. "That's when my husband dropped the phone and started screaming.

"I'm just hoping we find the bodies."

Samuel is believed to have told the children to open the back door of the van in order to escape but they were unable to do so before it sank.

The Pasadena Independent School District paid tribute to the children on its Facebook page.

It said: "Pasadena ISD's heart is heavy as we have learned that four Pasadena ISD students and two of their great-grand parents were swept away by the floodwaters of Greens Bayou while trying to escape the floodwaters of Harvey.

“Our thoughts and prayers are with the family, the Pasadena High School and Williams Elementary Communities during this most difficult time. Our deepest sympathy for the family.”

A Houston police officer also drowned in his patrol car after becoming trapped.

Officials expect the number to rise once waters recede and further searches are conducted.

Around 6,200 people are currently staying in shelters, having been evacuated from their homes as floods devastated the area and it is thought that a further 30,000 people could need to be given emergency accommodation.

The aftermath of Hurricane Harvey Show all 19 1 /19 The aftermath of Hurricane Harvey The aftermath of Hurricane Harvey A tattered U.S. flag damaged in Hurricane Harvey, flies in Conroe, Texas Reuters The aftermath of Hurricane Harvey Lisa Rehr holds her four-year old son Maximus, after they lost their home to Hurricane Harvey, as they await to be evacuated with their belongings from Rockport, Texas Reuters The aftermath of Hurricane Harvey People line up for food as others rest at the George R. Brown Convention Center AP Photo/LM Otero The aftermath of Hurricane Harvey Volunteers with The American Red Cross register evacuees at the George R. Brown Convention Center Reuters/Nick Oxford The aftermath of Hurricane Harvey Soldiers with the Texas Army National Guard help the residents of Cyprus Creek Reuters The aftermath of Hurricane Harvey Residents wade through floodwater Reuters/Nick Oxford The aftermath of Hurricane Harvey Residents walk along the flooded roadway of Texas 249 as they evacuate their adjacent neighborhoods EPA The aftermath of Hurricane Harvey A man floats past a truck submerged on a freeway flooded by Tropical Storm Harvey on Sunday AP The aftermath of Hurricane Harvey People are rescued by airboat as they evacuate from flood waters from Hurricane Harvey in Dickinson, Texas Reuters The aftermath of Hurricane Harvey James Archiable carries his bike through the flooded intersection at Taylor and Usenet near downtown Houston, Texas EPA The aftermath of Hurricane Harvey A massive sinkhole opened up on a motorway in Rosenburg, a city 25 miles southwest of Houston, Texas Rosenberg Police The aftermath of Hurricane Harvey People are rescued from flood waters from Hurricane Harvey in an armored police mine-resistant ambush protected vehicle in Dickinson, Texas Reuters The aftermath of Hurricane Harvey People are rescued from flood waters from Hurricane Harvey on a boat in Dickinson, Texas Reuters The aftermath of Hurricane Harvey Evacuees are airlifted in a US Coast Guard helicopter after flooding due to Hurricane Harvey inundated neighborhoods in Houston, Texas Reuters The aftermath of Hurricane Harvey Evacuees leave a US Coast Guard helicopter after being rescued from flooding due to Hurricane Harvey in Houston, Texas Reuters The aftermath of Hurricane Harvey Residents look on at a submerged motorway during a break in the rain in Houston, Texas EPA The aftermath of Hurricane Harvey People photograph the submerged motorway interchange EPA The aftermath of Hurricane Harvey Debris lies on the ground after a building was destroyed by Hurricane Harvey in Aransas Pass, Texas AP The aftermath of Hurricane Harvey Dominic Dominguez searches for his boat in a boat storage facility that was heavily damaged by Hurricane Harvey near Rockport, Texas EPA

Rescue shelters are reportedly overflowing and officials are desperately seeking alternative venues to house people who have been displaced.

The region received 30 inches of rain in just two days and had a year’s worth of precipitation dumped on it in the space of one week.

Harvey is the most powerful storm to hit Texas in 50 years. Officials warned the extreme weather is set to continue, although the worst is expected to move eastwards towards the state of Louisiana.

Donald Trump is due to visit Texas to survey the damage.