BANGKOK — The secretary general of the United Nations, Ban Ki-moon, warned the Myanmar government on Wednesday of “dangerous polarization” between Buddhists and Muslims and urged the leaders of the Buddhist-majority country to resolve the question of citizenship for the nearly one million stateless Muslims near the border with Bangladesh.

In prepared remarks made to diplomats in New York, Mr. Ban issued what appeared to be his strongest criticism to date of Myanmar’s handling of religious violence. Over the past year, Buddhist mobs have killed about 200 Muslims and forced more than 150,000 people, mostly Muslims, from their homes.

Mr. Ban said the government’s vows to protect lives and punish perpetrators needed to be “translated into concrete action.”

“If it is not addressed urgently and firmly, underlying tensions could provoke more upheaval, undermining the reform process and triggering negative regional repercussions,” Mr. Ban said. The government has won accolades for moving the country toward democracy after decades of military rule, but the growing violence has raised questions about the government’s ability and willingness to promote civic harmony and defend the rights of minorities.