Bangkok — A pro-democracy activist from Myanmar who is a naturalized American citizen has been released from prison a month after a court sentenced him to five years of hard labor, the United States Embassy in Yangon said on Thursday.The American, Mr. Nyi Nyi Aung, had been in detention since September, when he was arrested soon after arriving at the country’s main airport in Yangon.

Mr. Nyi Nyi Aung, who has spent the better part of the last decade campaigning for democracy in the military-ruled country, was due to arrive in Bangkok later on Thursday.

Family members said they believed that he had made the risky decision to travel to Myanmar to visit his mother, Daw San San Tin, who was detained for her involvement in September 2007 antigovernment protests. She has thyroid cancer and is serving a five-year term in a remote prison in Meiktila in central Myanmar.

Mr. Nyi Nyi Aung, who was born Kyaw Zaw Lin, was initially accused of inciting unrest but was ultimately sentenced to three years for forging a national identity card, one year for possession of undeclared foreign currency and one year for failing to renounce his Burmese citizenship after becoming an American citizen in 2002. At the time of sentencing, the judge said he could serve the sentences concurrently, effectively reducing the total jail time to 3 years.

A spokesman for the United States Embassy in Yangon, Drake Weisert, did not provide the reason for Mr. Nyi Nyi Aung’s release but said, “We welcome that development.”