US President Donald Trump has threatened a decade in jail to anyone caught damaging Washington’s Lincoln Memorial Reflecting Pool after the water turned green.

Disasters have marred the national landmark since Trump’s $US14.7 million ($21 million) renovation was completed earlier this month.

An algal bloom turned the water vivid green, and huge flaps of paint have been peeling from inside the pool.

Trump has claimed, without evidence, that people have been vandalising the monument and there has been a heavy police presence.

Five people were reportedly arrested, among them US Olympic canoeist David Hearn who said he was detained when he reached in and touched a “flapping” piece that had peeled off.

Earlier this week, National Parks workers poured hydrogen peroxide into the pool to deal with the algae.

“Of the MANY Statues and Fountains that we rebuilt, renovated, cleaned, and fixed, the only one that was Vandalised was the Reflecting Pool, which is being taken care of, ASAP!” Trump posted to Truth Social.

“It has been given a 300 foot long gash, chemicals have been illegally placed in the water.

“Please remember that there is a 10 year prison sentence for the destruction, or even the attempted destruction, of such things — Which will be fully enforced!”

White House press secretary Karoline Leavitt claimed those arrested were “deranged” Democrat supporters intent on “desecrating” the federal monument.

The DC Water authority issued a permit to drain the 609-metre rectangular pool, it said on Monday (local time), while the repair company said it would fix the pool as part of its warranty.

Peeling paint and algae growth have been visible in the pool since soon after Trump declared the renovation project complete on June 6.

Critics ‌have raised concerns about ‌the no-bid contract to ⁠recoat the pool before the nation’s 250th anniversary celebrations next month, as well as for ​the ducks that use its water.

On Monday, Trump echoed a weekend threat by US Attorney Jeanine Pirro to prosecute people accused of attempting to destroy the pool.

“Please remember that there is a 10-year prison sentence for the ⁠destruction, or even the attempted destruction, of such things —  Which will be ‌fully enforced!” ​Trump earlier wrote in a social media post.

Destruction of federal property can carry a maximum prison sentence of 10 years.

Trump told reporters ​on Monday that ‌the pool “looks very good”.

He accused someone of putting fertiliser in the water or taking action to create the algae, without ​providing evidence, adding that the algae was now dead and would be vacuumed out of the pool.

Trump also said there was a 90 metre “slit” through the pool, which he said was likely caused by a box cutter or knife of some kind.

“It’s not ​a ​lot of damage, but … we’ll probably have to let ​the water out and re-fix it. They went in there with ‌a knife,” he said, without providing evidence.

At least five people have been arrested, including a former Olympian who publicly denied the charges, while five others were issued citations, according to media reports citing an administration official.

The company responsible for the renovation work, Virginia-based Atlantic Industrial Coatings, said on Sunday the areas that required repairs made up “a ​very small part of the massive 7-acre (2.83-hectare) ⁠project, and do not indicate a failure of the liner”.

-with AAP