China has cooled to a landmark deal to curb emissions from international flights, with the country no longer listed as a participant in the agreement's first phase, according to the United Nations aviation agency's website.

China does not appear on a June 29 list of participants in the voluntary phase of the deal brokered by the International Civil Aviation Organization in 2016, according to ICAO's website.

Aviation powerhouse China was included in previous lists. It is not clear why China now objects to the deal. The pilot phase will begin in 2021.