The boy, who cannot be named because of his age, entered the plea via videolink at London's Central Criminal Court on Thursday.

He is charged with incitement to murder police officers at a Melbourne event marking the centenary of the Australian and New Zealand Army Corps (ANZAC) landings at Gallipoli during World War I. Prosecutors dropped a second charge relating to inciting beheading.

The boy was arrested in northwestern England in April. Days before he was taken into custody, Australian police detained five teenagers on suspicion of plotting what Australian Prime Minister Tony Abbott described as an "Islamic State" group-inspired attack on the April 25 ANZAC ceremony.

Prosecutor Paul Greeney told the court Thursday that the accused had sent thousands of mobile phone text messages to an 18-year-old Australian suspect in March, urging him to "to commit an act of terrorism abroad, namely the murder of police officers during an attack" on the parade.

One message said "suggest you break into someone's house and get your first taste of beheading."

The boy is due to be sentenced on September 3.

The landing at Gallipoli in 1915 was the first major military action involving troops from Australia and New Zealand during World War 1. At this year's 100th anniversary events, a crowd of around 85,000 people attended a ceremony at Melbourne's Shrine of Remembrance. A total of 131,000 troops died during the campaign from 1915 to 1916, including 86,000 Turkish soldiers, 25,000 British, 10,000 French, and 10,000 ANZAC troops.

nm/rc (Reuters, AP)