The National Guard has arrived at Port Aransas to help with the clean-up operation.

Earlier, equipment was moved to the area to help clear access to devastated parts of the city.

Pictures shared on social media show tractors clearing mud and debris from roads to allow emergency services through.

Governor Abbott praised the White House for "stepping up" to sign the disaster declaration.

Other key points he made:

• More than 1,000 people are involved in the search-and-rescue operation
• 1,800 military will help with the clean up
• The Red Cross has opened 21 shelters, and it has 42 more on standby.

Governor Abbott has been asked if he can confirm any fatalities.

He said, "No".

Recovery efforts and the clean-up in the wake of Hurricane Harvey will take "years", officials say.

"This is going to be an unprecedented, long and frustrating event for the state of Texas," the head of the US Federal Emergency Management Agency, Brock Long, told MSNBC.

"The recovery from this disaster is going to be years", Mr Long added.

The storm has destroyed homes, severed power lines and forced tens of thousands of people to flee.

Governor Abbot says the state disaster declaration - which was signed last night by President Trump - has now be expanded to include 20 more counties.

Texas Governor Greg Abbott is giving a press conference. He says the current focus is on the rescue mission. He says some areas have already experienced 20 inches of rain, and there are concerns that there will be 20 or 30 inches more.

Hurricane Harvey has weakened in wind strength since landfall and has now been downgraded by the US National Hurricane Center to a tropical storm.

Meanwhile, pictures emerging from Port Aransas, a city on Mustang Island, near Corpus Christi, show mobile homes stacked against one another, cars turned upside down in the street and boats lining the pavement.

The mayor of Rockport, Patrick Rios, says the town has suffered widespread devastation with the collapse of several buildings, leading to fears that residents may have been buried under the rubble.

Elsewhere in the state, water levels are continuing to rise and rivers are spilling over. Emergency services said a school, a hotel and other buildings had also suffered structural damage.

Image source, EPA

More images are coming in of structures hit by the storm - here are a few.

Image source, Reuters

Image source, Reuters

Image source, Reuters

Speaking to the BBC, the deputy director of the National Hurricane Center, Dr Mark DeMaria, said the flood risk in affected areas was very high.

It's been more than 12 hours since landfall, so here's the latest:

• Hurricane Harvey touched down as a category four storm, but is now down to category one and will likely be downgraded to a tropical storm soon
• The hurricane is now moving very slowly - bad news, as it will drop more rain as it stalls
• The port city of Rockport appears to have been badly hit by the storm, but there is no word on casualty numbers yet
• At least 300,000 customers are without electricity
• Weather officials warn of "catastrophic" rainfall over the coming days

The BBC's James Cook, who has been reporting from Corpus Christi, has just shared pictures of severely damaged buildings in Rockport.

He earlier reported of serious concern for the town, where the roof of a housing centre for elderly people collapsed trapping a number of residents.

Ten people were later taken to a country jail for treatment, AP news agency reports, citing local TV station KIII.

The town of Rockport, where Hurricane Harvey made landfall in the early hours of Saturday, has been severely battered by winds of up to 130mph (215km/h).

The latest pictures from the city show buildings destroyed as local residents return to assess the damage caused to their homes.

Image source, Getty Images

Image source, Getty Images

Images also show a local community church reduced to rubble.

We'll bring you the latest updates from governor Greg Abbott as soon as we can - he'll be speaking in the next few minutes.

Prison officials say about 4,500 inmates are to be moved from three prisons in Brazoria County, south of Houston.

The level of the Brazos River is rising significantly, so the inmates are being moved to other prisons in east Texas.

The Caller-Times newspaper in Corpus Christi reports, external that the main hospital there is taking emergency admissions only.

It adds:

Some of the worst damage so far appears to be in Katy, Texas. One woman there who lost her home was interviewed by CBS News.

While the biggest hurricane in 13 years made landfall in Texas, you could be forgiven for not noticing other important news stories taking place.

The timing of two in particular - President Trump's pardoning of a controversial sheriff and his decision to reinstate a ban on transgender members of the military - has been criticised.

Democrat Chuck Schumer, the Senate minority leader, attacked Mr Trump's timing on Twitter.

The Caller-Times newspaper in Corpus Christi is reporting, external that three babies were born in the city while the hurricane passed nearby.

Two were delivered by Caesarean section in hospital, and another was a home birth. The mothers and children are all doing well, a medical official said.