There is no "plan B" for Myanmar if the government does not accept U.N. recommendations for a resolution to Rohingya crisis, former U.N. secretary general Kofi Annan told reporters in New York on Friday.

Annan, who is chairing a commission on Myanmar's Rakhine state, briefed the Security Council as it discussed ways to halt the humanitarian crisis that has seen more than half a million Rohingya Muslims flee to Bangladesh.

Myanmar is going through a "difficult transition" after five decades of military rule, but if the international community cannot work together on a plan based on his recommendations, "we are going to have a long-term festering problem," Annan said.

"I don't have a plan B - my work is done," Annan said of his commission's findings. "We have to tackle the root causes and the report deals with that."

The Security Council and Secretary General Antonio Guterres have repeatedly called on Myanmar's government to stop the violent military "clearance operations," ensure humanitarian access and allow Rohingya refugees to return home.

"This is not going to be easy - they will only go back if they have a sense of security and confidence their lives will be better," Annan said.

Myanmar security forces have carried out "well-organized, coordinated and systematic" attacks against the Rohingya, according to the U.N.'s Human Rights Office.

It is a positive step that Myanmar's authorities are softening rules, which state refugees need to show proof of residency to return home, as "it's very hard to have that if you left a burning village under attack," British ambassador Matthew Rycroft said.

Britain and France are leading Security Council negotiations for a resolution to the crisis.