A memorial in Sydney's west to the hundreds of thousands of Assyrians who died in World War I has been trashed by vandals who spray-painted swastikas and offensive slogans on the monument.

The monument, erected by Assyrian leaders in Bonnyrigg, refers to the killing of 750,000 Assyrians by Ottoman troops as "genocide".

Monument to the hundreds and thousands of Assyrian who died in World War I in Bonnyrigg before it was vandalised. Credit:Wolter Peeters

Early Wednesday morning the words "F--- Armenians, Assyrians and Jews" were found scrawled on the monument.