Taipei, Mar 12 (IANS): The US Ambassador for Religious Freedom said on Tuesday that his country's government was very concerned over the imprisonment of a Taiwanese activist in China and called for his release.

The US envoy, who was attending a forum on religious freedom in the Indo-Pacific region in Taipei, had met Lee Ming-che's wife, Lee Ching-yu, when she visited the US to mobilize international support for her husband's release.

"We call on the government of China to release him back to her and Taiwan," Ambassador Sam Brownback said at a press conference.

Lee Ming-che, a Taiwanese pro-Democracy activist, disappeared mysteriously in March 2017 while he was travelling in mainland China, Efe news reported.

He had disappeared after entering China from Macao on a private visit and his whereabouts had remained unknown until May when Chinese authorities said they had detained him on suspicions of subversion of state power.

Meanwhile, Chinese Foreign Ministry spokesperson Lu Kang on Tuesday criticized Brownback for interfering in Lee's case, calling it "China's internal matter" and one involving parties on either side of the Taiwan Strait.

"Others have no right to meddle," said the spokesperson, adding that China was opposed to any official contact between the US and Taiwan.

During his trial in September, Lee had pleaded guilty to charges of subversion, according to video footage released by the Chinese court.

Lee admitted to cooperating with Peng Yuhua, a Chinese citizen, who had established several discussion groups critical of the government, to intentionally disseminate information and articles attacking the Communist system and the Chinese government.

Lee, a former employee of Taiwan's ruling Democratic Progressive Party, was a professor at the Wenshan Community College in Taipei and a volunteer at the Covenants Watch NGO.

In Taiwan, Lee's trial was considered a warning from Beijing to pro-Democracy activists or dissident voices on the island.

On Monday, Brownback had criticized China for practicing "religious persecution".

The US maintains diplomatic ties with China, but at the same time it is Taiwan's main military ally, and according to a 1979 treaty, is committed to supply arms to boost its defence capabilities.