Kyran Durnin was last seen in Drogheda, County Louth, on 30 August, and is presumed dead, Irish police said.

"Despite extensive enquiries carried out to date, we have been unable to either locate Kyran, identify any information on his current whereabouts or any evidence that he is currently alive," they said in a statement.

"Following enquiries to date, investigating police now believe that Kyran is missing, presumed dead.

"A murder investigation has been commenced."

No arrests have been made.

Ireland's Child and Family Agency, Tulsa, is working closely with officers.

Police in Drogheda appealed for anyone with information to get in touch.

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"Any information, no matter how insignificant it may seem, will be welcomed by the investigation team. This information will be treated in the strictest confidence," a spokeswoman said.