SYDNEY (Reuters) - An Australian teenager on Thursday pleaded guilty to planning a terror attack which involved beheading a police officer and attaching explosives to a kangaroo at commemorations of the ANZAC landings at Gallipoli during World War One.

Sevdet Ramadan Besim, 19, had planned to attack police at the Melbourne ANZAC day parade on April 25, 2015 but his scheme was uncovered by police in Britain who found messages on the phone of a 15-year-old British boy to a man in Australia.

The British boy last year pleaded guilty to inciting an attack on an ANZAC day parade in Melbourne, prosecutors said.

Besim was not immediately sentenced but faces a life term in prison.

ANZAC Day, April 25, is a major annual holiday in Australia and New Zealand marking the first major battle involving troops from both countries during World War One at Gallipoli in Turkey.

(Reporting by Colin Packham; Editing by Michael Perry)