WASHINGTON -- Federal officials have yet to respond to two United Nations human rights envoys who formally requested that the U.S. government protect Occupy protesters against excessive force by law enforcement officials.

In a letter to Secretary of State Hillary Clinton, the two envoys called on U.S. officials to "explain the behavior of police departments that violently disbanded some Occupy protests last fall" and expressed concern that excessive use of force "could have been related to [the protesters'] dissenting views, criticisms of economic policies, and their legitimate work in the defense of human rights and fundamental freedoms."

The envoys also reminded the U.S. government of its international obligations to "take all necessary measures to guarantee that the rights and freedoms of all peaceful protesters be respected."

The letter, from Frank La Rue, who serves as the U.N. special rapporteur for the protection of free expression, and Maina Kiai, the special rapporteur for freedom of peaceful assembly, was sent in December 2011.

It was publicly released last week in connection with the 20th annual U.N. Human Rights Council meeting, which started Monday and at which both rapporteurs -- independent experts sent out to investigate human rights problems around the world -- will make their annual reports.

The U.S. government has not answered the letter. A State Department spokeswoman told HuffPost that "the U.S. will be replying," but she couldn't say when or how. "We do not comment on the substance of diplomatic correspondence," she said.

"The Department of Justice's Civil Rights Division is the lead agency for violations of human rights or civil rights in the United States" and therefore will have input into the response, the spokeswoman noted.

U.N. officials could not be reached for comment.

"Lack of an answer does not make the U.S. look good in the international community," said Jamil Dakwar, director of the American Civil Liberties Union's human rights program.

"The U.S. should at a very minimum respond to a letter like this," he said. "And if they believe that law enforcement agencies operated under legal, constitutional authority and there were no problems, then they should explain that and present that" before the Human Rights Council.

The rapporteurs' letter described how groups of peaceful Occupy protesters were forcibly removed from their encampments in various U.S. cities, including New York, Seattle, Denver and Oakland, Calif., last fall.

"In the conduct of such operations, law enforcement officials in these cities allegedly used violence as a means to forcibly remove unwilling protesters from the public areas in which they were located," the letter said. "In some instances, police allegedly used force unnecessarily and disproportionately," including "pepper spray and tear gas … used deliberately on protesters at a very close distance."

The letter asked the State Department to respond to several questions about what happened and what sort of follow-up there was. It inquired, "What was the legal basis for these actions that limit the exercise of the legitimate rights of the protesters?"

The letter was written shortly after La Rue told HuffPost that he would contact the U.S. government demanding to know why federal officials were not protecting the rights of Occupy demonstrators. La Rue's view was that the federal government's role was to ensure that local governments respected protesters' human and constitutional rights.

In reality, the Department of Homeland Security helped coordinate the crackdowns, sharing information gathered by its nationwide monitoring and surveillance network with local governments.

In the letter, the envoys raised a particular concern that the "crowd control techniques used to manage and disperse these assemblies might have been intended to insert fear and intimidation on protesters throughout the country."