More than 30,000 people are expected to be placed in temporary shelters in the US state of Texas due to widespread flooding caused by Tropical Storm Harvey, US officials said, with more rain expected in the coming days.

Brock Long, director of the Federal Emergency Management Agency, said on Monday that 50 counties in Texas are affected by the floods, which were brought about by an estimated six months-worth of rain falling in the last three days alone.

"We have not seen an event like this. You could not draw this forecast up, you could not dream this forecast up," Long said.

Earlier, Texas Governor Greg Abbott deployed an additional 1,000 National Guard troops, on top of the 3,000 already sent in the flood-stricken state, which is the size of France, Belgium and Switzerland combined.

READ MORE: Facing Hurricane Harvey

Texas officials said on Monday that six more people are feared to have died in the Houston area as a result of the storm and flooding.

Tricia Bentley, spokeswoman for the medical examiner's office in Harris County - which includes the city of Houston - confirmed six deaths since Sunday that are "potentially tied to Hurricane Harvey".

She was unable to specify the causes of death, explaining that "some people may have had a medical emergency and just could not seek help because of the flooding".

The fatalities were distinct from unconfirmed reports that a family of six had died in a van caught in floodwaters.

Three people were previously confirmed to have died after Harvey hit Texas on Friday. In Houston, a woman drowned when she left a car that had stalled in high water. Another man was found dead in a flooded Wal-Mart car park in Galveston County.

Thousands seek shelter from floods after Harvey

And one person was killed when a house caught fire in the Rockport area, where Harvey made landfall.

Al Jazeera's Heidi Zhou-Castro, reporting from Houston, the largest city in Texas, said that in the last 48 hours, emergency agencies have received some 6,000 calls for help.

She said that between 300 to 400 households were still waiting to be reached by rescuers as of 13:00 GMT on Monday.

Our correspondent also said that the flooding is expected to rise in some parts of Houston, as authorities are expected to open dams and levies in the area, to ease pressure from the continuous rain.

Al Jazeera's weather presenter Richard Angwin reported that the "worst is probably yet to come", saying that the storm, which is moving only at an estimated five kilometres per hour, could dump more rain in the coming days.

"We are only half-way through the rainfall," he said.

"The storm is going nowhere very fast. It is going so slowly. There's still a lot still coming out of it, picking up warm water in the Gulf of Mexico."

'Catastrophic'

In some areas just north of Houston, rainfall has already reached 1,000mm in the last three days, which our correspondent described as "catastrophic".

Aside from the 50 counties affected in southern Texas, a portion of the south-western state of Louisiana was also affected by Harvey, which made landfall late on Friday as a Category 4 hurricane.

Harvey has since been downgraded to a tropical storm, but the damage it has left is extensive.

About 450,000 people are anticipated to apply for disaster assistance, which would make them eligible for financial support, possible replacement of property and other disaster-related aid.

President Donald Trump is expected to visit Texas on Tuesday, but not Houston as flooding in the US' fourth-largest city continues.

Harvey is the first major natural disaster to hit the United States during Trump's presidency. On Sunday, he convened a cabinet meeting by telephone in response to the disaster.

Harvey was the fiercest hurricane to hit the country in 13 years, and the strongest to strike Texas since 1961's Hurricane Carla, the most powerful Texas hurricane on record.