The United States said it condemned any use of phosphorus as a crowd control measure after an official Myanmar report Tuesday found the toxic agent had been used to end a rally at a copper mine.
Dozens of people, including monks, were injured in the November clampdown, many suffering burns from the white phosphorus, according to a parliamentary report led by opposition icon Aung San Suu Kyi.
"We have opposed the use of phosphorus as a crowd control agent, and we have urged the government to ensure that its security forces exercise maximum restraint," State Department spokeswoman Victoria Nuland said.
"This phosphorus... can be quite damaging to humans. There are other crowd control elements that are better suited."
The burns sustained by the protesters caused a wave of outrage across Myanmar, sparking further protests and leading to an official apology to senior clerics for a crackdown widely seen as a throwback to junta-era tactics.
The clampdown at the mine in northern Nomywa came in the same month that US President Barack Obama made a landmark visit to the Myanmar, seeking to encourage the startling reforms launched in the former pariah state.
US relations with Myanmar, which had languished for decades under the Southeast Asian nation's military leadership, are now on track to being normalized with hardly any sanctions remaining against the country.
But Nuland admitted "it's no secret that this is a work in progress in Burma" referring to the country by its former name, adding that Washington continued to monitor the situation in the country closely.
"As we said from the beginning, we would take steps to open our relationship as they take steps," Nuland told reporters.
"But we are continuing... to have a rigorous human rights dialogue with Burma, which includes issues like police conduct, freedom of assembly, these kinds of things."