Washington: Hurricane Harvey, the biggest storm to hit the US mainland in 13 years, slammed into Texas spawning tornadoes, flooding, fires and widespread power outages across the state on Saturday.

The storm, which blasted ashore as a Category 4 around 11 p.m. on Friday between Port Aransas and Port O'Connor, was a Category 1 by mid-morning Saturday, packing winds of 80 mph as it plodded northwest, reported Fox News.

Millions of coastal residents in south Texas experienced hurricane-force winds that downed trees, power poles and signs. More than 213,000 people were currently without power on the Texas Gulf Coast, the Electric Reliability Council of Texas said.

Coastal cities remained in danger of a potentially deadly 13-feet storm surge, with places even far inland predicted to get 40 inches of rain through Wednesday. The city also cancelled two big concerts of British band Coldplay and singer Mary J. Blige.

A massive fire destroyed at least three homes on Texas' Bolivar Peninsula late Friday night. Several residents are feared trapped in collapsed buildings, reports said.

According to forecasters, the impact of Hurricane Harvey, the strongest to hit the US since Hurricane Charley in August 2004, will be devastating and leave areas "uninhabitable for weeks or months".

Harvey wielded the "highest potential to kill the most amount of people and cause the most amount of damage," Brock Long, director of the Federal Emergency Management Agency, told CNN.

There were reports of structural and building problems in Rockport, Aransas Pass, and Port Aransas. Harvey made a double landfall: Northeast of the city of Corpus Christi initially, then just north of Rockport a few hours later.

Emergency services in Rockport said a school, a hotel, a housing complex for the elderly and other buildings suffered structural damage.

At least 10 people were injured when the roof of a senior housing centre in Rockport caved in late Friday. In Aransas Pass area, a part of the Harbour Master Building was seen flying away.

The Rockport Mayor advised residents who refused to evacuate to write their names and Social Security numbers on their forearms to "help out first responders should they find a body".

Hospitals had evacuated patients ahead of the hurricane. President Donald Trump, who is likely to visit Texas early next week, signed a disaster declaration for the city and praised emergency services in a tweet.

"You are doing a great job - the world is watching! Be safe," Trump tweeted.

Production on oil rigs in the Gulf of Mexico remained halted, coastal refineries remained idle and thousands of cruise ship customers waited offshore until their vessels were able to dock safely.

Governor Abbot said it was obvious Texas was confronting a "very major disaster", and more than 1,000 National Guardsmen were to help with the disaster relief operation.