Residents living under an ice dam in Alaska have been ordered to evacuate as snowmelt threatens to flood the US state’s capital.

The Mendenhall Glacier, which acts as a huge dam for rainwater and snowmelt, has started to release, supercharged by climate change.

Officials urged people in parts of the state capital of Juneau, a city of 32,000, to evacuate ahead of what could be a record surge of floodwater.

In recent days, people in the flood zone were warned to be ready to evacuate.

Water had started escaping the ice dam on Tuesday (local time), with flooding expected on Thursday (AEST).

Debris from a home that partially fell into the Mendenhall River sits on its banks in Juneau. Photo: AAP

On Thursday morning (AEST), officials said emergency barriers installed throughout Juneau seemed to be funnelling water away from neighbourhoods, reported USA Today.

“The area will remain in major flood stage area for several more hours,” city officials said in a statement.

The Mendenhall Glacier is about 19 kilometres from Juneau and is a popular tourist attraction due to its proximity to Alaska’s capital city and easy access on walking trails.

The glacier acts as a dam for Suicide Basin, which fills each spring and summer with rainwater and snowmelt.

Homes on the city’s outskirts are within kilometres of Mendenhall Lake, which sits below the glacier. Many front the Mendenhall River.

The water being released in the glacial outburst is flowing into the river, putting homes that are closest to the river at risk. The US National Weather Service said it expected flooding to peak at 10am AEST Thursday.

“This will be a new record, based on all of the information that we have,” weather service meteorologist Nicole Ferrin said.

Flooding in a neighbourhood of Juneau. Photo: AAP

Flooding from the basin has become an annual concern, and in recent years has swept away houses and swamped hundreds of homes.

US government agencies installed temporary barriers this year in hopes of protecting several hundred homes in the inundation area from widespread damage.

The threat of so-called glacier outburst flooding has troubled parts of Juneau since 2011. In some years, there has been limited flooding of streets or properties near the lake or river.

This year’s flooding was predicted to peak at up to 5.12 metres.

Last year, nearly 300 residences were damaged.

A large outburst can release some 56 billion litres of water, according to the University of Alaska Southeast and Alaska Climate Adaptation Science Centre. That’s the equivalent of nearly 23,000 Olympic-size swimming pools.

City officials responded to concerns from property owners this year by installing a temporary levee along roughly four kilometres of riverbank in an attempt to guard against widespread flooding.

-with AAP