The Arakan Rohingya Salvation Army extremist terrorists launched attacks on police outposts in Myanmar's Rakhine State on August 25 previous year displacing a vast number of residents to border areas with Bangladesh.
The UN described the military's actions as "ethnic cleansing".
The government said its forces were engaged in a legitimate security campaign in response to a string of Rohingya insurgent attacks on the security forces.
The military was "taking harsh and stronger actions against such offenders", he said.
The UN spokesperson added that over the weekend, the Council delegation visited Bangladesh's Cox's Bazar, where they listened to testimony from refugees at the Kutupalong Camp.
Myanmar should move forward with the repatriation of the refugees according to its agreements with Bangladesh, she said.
Ambassadors from the UN Security Council probing Myanmar's crisis over its ethnic Rohingya Muslim minority met Monday with the country's leader and military chief after visiting Bangladesh, where about 700,000 Rohingya who fled military-led violence live in refugee camps.
When the diplomats met with Min Aung Hlaing, they asked him about the security situation in Maungdaw and Buithdaung - two of the three townships where the crackdown occurred - the repatriation process, projects on which Myanmar can work with United Nations agencies in Rakhine, and the military's willingness to act in accordance with the law in the state.
"She said what had happened or what was alleged to have happened to some of the Rohingya villagers was not acceptable and that if evidence were available it should be reported to the Burmese authorities and they would investigate", said Pierce.
"We are determined to find an end and a solution for this crisis" he said.
Suu Kyi's civilian government has no control over the military.
The Myanmar mission to the United Nations did not immediately respond to a request for comment on Pierce's suggestion.
Myanmar denies the Rohingya citizenship and accompanying rights. "But the news cycle had kind of died out until Bob Rae's (Canada's special envoy to Myanmar) came out and drummed up some more support again, so it's flawless timing".
"I do not see any resistance from China and Russian Federation".
The delegation was composed of permanent representatives and member states of the UN Security Council and representatives from immediate neighbours of Myanmar, including the current Asean Chair. The envoys passed two bulldozed villages near the camp.
In Rakhine, data from the U.N. Operational Satellite Applications Programme (UNOSAT) has shown hundreds of villages once inhabited by the Rohingya have now been burnt down. They are denied freedom of movement and other basic rights.
Bangladesh and Myanmar signed a repatriation agreement in November but no refugees have returned so far.
"We can not remain silent about it, and when we go back to NY...we will try to explore ways and means to speed up the implementation of the agreement signed between Bangladesh and Myanmar for [the] safe, free, and voluntary and dignified return of the refugees", he said.
Myanmar and Bangladesh agreed in January to complete the voluntary repatriation of the refugees within two years but differences between the two sides remain and implementation of the plan has been slow.
The UN refugee agency and Bangladesh recently finalized a memorandum of understanding that said the repatriation process must be "safe, voluntary and dignified ... in line with global standards".
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