The citizenship law recognizes eight races and 130 minority groups, but omits the country's 800,000 Rohingya. Many Myanmar Buddhists view the Rohingya as interlopers brought in by British colonialists from what is now Bangladesh, but many Rohingya say they have lived in the country for hundreds of years.
The Rohingya speak a Bengali dialect and resemble Bangladeshis, with darker skin than most people in Myanmar. Bangladesh also refuses to accept them as citizens.
The Rohingya issue is a highly sensitive one in Myanmar, which is overwhelmingly Buddhist. Ultra-nationalists have widely promoted anti-Rohingya propaganda, which has triggered prejudice against Muslims in general.
Opposition leader Aung San Suu Kyi has been lukewarm in confronting the issue, even though she was embraced as a heroine of human rights when fighting against Myanmar's previous military regime.
Suu Kyi, who has expressed interest in becoming the country's next president, declined to meet with an Organization of Islamic Cooperation delegation which visited Myanmar recently to look into the plight of the religious minority.
A spokesman for Suu Kyi's National League for Democracy party, Nyan Win, criticized the U.N. resolution.
"Even the United Nations is interfering in Myanmar's internal affairs," he said.