The pigeons, caught in central Vietnam, had Chinese characters stamped on their wings with red ink and a mysterious ring around their ankles.

A total of 16 of the pigeons have been caught since last November by residents of Da Nang City, the local Thanh Nien News website reports.

Colonel Tran Phuoc Hung of the Da Nang Police Department told the news outlet that police had traced the pigeons based on the markings on their wings, which are their club registration numbers and their racing names.

Some local newspaper reports quoted residents as saying that the rings contained something which appeared to be a memory card.

The pigeons actually belonged to racing clubs based in Taiwan, China, Indonesia, the Philippines and Vietnam itself, having abandoned racing competitions.

Tensions rose between Vietnam and China last year after after China began to extract oil in what Vietnam says it is exclusive economic zone in the South China Sea, referred to in Vietnam as the East Sea.

At the time of the incident the Vietnamese Coastguard released footage showing what they said were Chinese vessels intentionally ramming Vietnamese ships.

More than 20 people were killed in anti-China riots last May after the incident, which observers described as one of the worst breakdowns in Sino-Vietnamese relations since the neighbours fought a brief border war in 1979.

By Jon Stone - The Independent - January 16, 2015