A flood-hit southeast Texas city lost its drinking water supply and police and soldiers rescued thousands still stranded on Thursday after powerful storm Harvey killed 44 people and displaced more than a million on the Gulf Coast.

Scientists say the devastating intensity of hurricanes such as Harvey is consistent with global warming trends -- rising seas, warming oceans, hotter air -- and warn of "bigger and stronger" storms to come.

Floodwaters from Tropical Storm Harvey, which has already killed at least seven people in Texas and was expected to drive tens of thousands from their homes, are likely to rise, officials warned on Monday, as heavy rain continued to pound the US Gulf Coast.

Catastrophic flooding triggered by Tropical Storm Harvey inundates Houston, forcing residents of the fourth most populous US city to flee in boats or hunker down in anticipation of several more days of "unprecedented" rainfall.

Hurricane Harvey slammed into the Texas coast late Friday, unleashing torrents of rain and packing powerful winds as it became the first major storm to hit the US mainland in 12 years.