The international rights group said on Tuesday that Myanmar's security forces have committed "serious violations" that include rape and other sexual violence, murder, forced deportation and persecution of the Rohingya.
"The Burmese [Myanmar] military is brutally expelling the Rohingya from northern Rakhine State," James Ross, legal and policy director at Human Rights Watch, told Reuters news agency.
"The massacres of villagers and mass arson driving people from their homes are all crimes against humanity," he said.
In the past, HRW has also accused Buddhist monks in the area, as well as Myanmar security forces of "crimes against humanity".
"Attaching a legal label to the ghastly crimes ... may seem inconsequential," Ross said.
"But global recognition that crimes against humanity are taking place should stir the UN and concerned governments to action."
Government spokesman Zaw Htay rejected HRW's accusations, saying no other Myanmar government was as committed to the promotion of rights as much as the current one.
"Accusations without any strong evidence are dangerous. It makes it difficult for the government to handle things," he told Reuters news agencies.
Last week, HRW called for the UN Security Council to impose an arms embargo and sanctions on the Burmese military to end its "ethnic cleansing campaign" against the Rohingya.
Nearly 440,000 refugees have fled to Bangladesh since Myanmar’s army launched a military campaign in response to an attack by Rohingya fighters on dozens of police posts and an army base on August 25.
The UN has called the Rohingya exodus from Myanmar to Bangladesh "the most urgent refugee emergency in the world".
The mainly Muslim minority, who live primarily in Rakhine State, is not recognised as an ethnic group in Myanmar, despite having lived there for generations.
They have been denied citizenship and are stateless.
Many in Buddhist-majority Myanmar consider the Rohingya "Bengali", rejecting the term Rohingya as a recent invention.
Source: Al Jazeera and news agencies