Images showed a stretch of peninsular Malaysia's second-longest river, Sungai Perak, in a bright shade of turquoise, raising fresh concerns over pollution and mining oversight.
It was not clear what caused the river to change colour, but Johari Abdul Ghani, Malaysia's natural resources minister, said an investigation was looking into chemicals used in treatment processes at a rare earths mine.
Johari told parliament earlier this week that the government had ordered work to be suspended for three weeks at the rare earths facility and two tin mines near that part of the river.
Environmental group Greenpeace Malaysia called for "full transparency and an independent investigation".
The group said in a statement it was "deeply concerned about the Perak River turning bright blue and the subsequent suspension of mining operations".
"The decision to halt operations is necessary, but it is only a first step" that must lead to a full investigation, Greenpeace said.
Johari said readings taken at the rare earths site measured at 13 becquerels -- a radiation unit -- far above the one-becquerel limit stipulated in the operation's environmental impact assessment report, according to national news agency Bernama.
Greenpeace said authorities also needed to check water quality downstream.
The water at the Sungai Perak River first turned blue a month ago, Bernama reported.
Malaysia is becoming a significant player in rare earth mining and processing.
US President Donald Trump and Malaysian Prime Minister Anwar Ibrahim last month inked a trade agreement, boosting US access to the critical minerals.
Malaysia said in 2023 that it holds around 16.2 million tonnes of untapped rare earth reserves.
A nationwide moratorium on raw rare earth exports took effect on January 1, 2024, aiming to encourage domestic processing.
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