The downtrodden US city of Flint was poisoned in a misguided drive by penny-pinching officials to save money, a lawsuit filed Wednesday claimed, demanding that corroded lead pipes that contaminate its tap water be immediately replaced.

Officials are accused of ignoring months of health warnings about foul-smelling and discolored water, even as residents complained it was making them sick.

"In a failed attempt to save a few bucks, state-appointed officials poisoned the drinking water of an important American city, causing permanent damage to an entire generation of its children," Michael Steinberg, legal director of the American Civil Liberties Union of Michigan, said in announcing the lawsuit.

"The people of Flint cannot trust the state of Michigan to fix this man-made disaster and that is why court oversight is critically needed."

Governor Rick Snyder -- who faces calls to resign over his handling of the scandal -- appointed a team of outside experts Wednesday to help the state resolve Flint's water crisis and deal with the long-term health impacts.

In an interview with CNN Wednesday he admitted the number of children harmed by lead in the water is likely much higher than tests have so far revealed.

"There could be many more," the governor told CNN, "and we're assuming that."

About 100 children have tested positive for elevated blood lead levels. But since those tests only show recent exposure, Snyder said officials are concerned that the problem is much more widespread.

Lead exposure is harmful to everyone. But it can have devastating impacts on young children by irreversibly harming brain development. It has been shown to lower intelligence, stunt growth and lead to aggressive and anti-social behavior.

Water treatment plants across the United States are required to closely monitor lead levels in tap water and use chemicals to reduce acidity and coat pipes to prevent corrosion.

Snyder vowed at a news conference to help "address the damage that's been done" in the predominantly poor and black city of 100,000.

But he stopped short of promising to replace the pipes, which began leaching lead after Flint switched to a cheaper but dangerously corrosive water supply.

"It's a lot of work to take out pipes, to redo the infrastructure," Snyder told reporters.

"The short-term solution is to hopefully recoat, and have it validated by third parties so we know the water is safely coming out."

- $1.5 billion fix -

Flint's mayor has estimated that the cost of fixing the damage done to the city's infrastructure by the corrosive water could reach $1.5 billion.

The cash-strapped city was reportedly hoping to save $5 million over two years by drawing water from the Flint River beginning in April 2014 rather than continuing to buy it from nearby Detroit.

The state's environment department approved the switch even though the city's treatment plant was not able to produce water that met state and federal standards.

It cost $12 million to switch Flint back to the Detroit water system in October after a local pediatrician released a study showing that the number of children with elevated blood-lead levels had doubled since the city began using river water.

- Nation's pipes need replacing -

Activists and environmentalists says the state now needs to spend whatever it takes to make sure the water is safe to drink.

"For years the state told us we were crazy, that our water was safe, which wasn't true," said Melissa Mays of Water You Fighting For, a Flint-based organization which joined the ACLU and the Natural Resources Defense Council in filing the lawsuit.

"For the sake of my kids and the people of Flint, we need a federal court to fix Flint's water problems because these city and state agencies failed us on their own."

Replacing all the lead pipes in Flint would take years and cause major disruption for residents because roads would need to be shut down to dig them out of the ground, said Greg DiLoreto of the American Society for Civil Engineers.

But while short-term fixes might be able to resolve Flint's lead problems for now, replacing those pipes is something that Flint -- and most other American cities -- has to start planning for, he told AFP.

A large proportion of the nation's water systems were built in the early 20th century and some pipes date back to the late 1800s.

"No engineer designed any system to last 150 years," DiLoreto said in a telephone interview.

"This is like your house. At some point you're going to have to put a new roof on it."