Story highlights UN human-rights chief had previously warned of "crimes against humanity" in Myanmar

Myanmar military rejected a proposed ceasefire with militants on Sunday

(CNN) The continuing Myanmar military operation against the minority Rohingya people appears to be a "textbook example of ethnic cleansing," the UN human rights chief said Monday.

United Nations High Commissioner for Human Rights Zeid Ra'ad Al Hussein said the full humanitarian situation in Rakhine state couldn't be fully assessed because of Myanmar's refusal to give access.

"Last year I warned that the pattern of gross violations of the human rights of the Rohingya suggested a widespread or systematic attack against the community, possibly amounting to crimes against humanity," he said in his opening statement at the Human Rights Council 36th session.

At least 313,000 Muslim Rohingya have fled across the border to Bangladesh since fighting broke out on August 25, according to the Inter Sector Coordination Group (ISCG) in Bangladesh, which represents UN and aid agencies.