WASHINGTON - More than 50 US lawmakers urged Myanmar's military rulers in a letter released Friday to immediately free US citizen Kyaw Zaw Lwin, also known as Nyi Nyi Aung, from prison amid health worries.

"We urge you in the strongest possible terms to immediately and unconditionally release Mr Aung and allow him to return to the United States," they wrote in the message to Myanmar junta leader Than Shwe.

The lawmakers said the charges against Aung were a pretext to hold him and that his "longstanding non-violent activities in support of freedom and democracy" in Myanmar were the real reason for his imprisonment.

"The detention of an American citizen under these circumstances has caused alarm among many members of the United States Congress, and raises serious doubts about your government's willingness to improve relations," they said.

Dissident groups from Myanmar, still known in Washington as Burma, have said Nyi Nyi Aung is a democracy activist and was hoping to see his ailing mother, herself detained over political activities, when he was arrested September 3.

His lawyers say he was deprived of food, sleep, medical treatment and US consular access in his first two weeks of detention.

His fiancee and his Washington-based lawyer have said that US diplomats have been prohibited from seeing Aung, and that his health has been deteriorating.