A video apparently released by the self-styled "Islamic State" (IS) militant group on Sunday should be treated as a tool of propaganda, British Prime Minister David Cameron said in a statement issued by his spokeswoman on Monday.

"We are examining the content of the video and the prime minister is being kept updated on that," said Helen Bower, Cameron's spokeswoman.

"It serves as a reminder of the barbarity of Daesh and what the world faces with these terrorists. It is also clearly a propaganda tool and should be treated as such," Bower added, referring to the group by its Arabic acronym.

The video shows a young boy - believed to be around four to five years old - standing alongside an older masked man, who is heard speaking English with a British accent.

The masked man addresses the camera in a manner reminiscent of Mohammed Emwazi (commonly dubbed "Jihadi John" in the UK press), a British citizen reportedly killed in a US-led airstrike in November. He said that the "Islamic State" would one day "invade" the UK and rule the country through "Shariah," a form of Islamic jurisprudence.

"This is a message to David Cameron," the man said in the roughly 10-minute video. "We will continue to wage holy war … and one day invade your land, where we will rule by the Shariah."

In December, the UK parliament approved Cameron's request for authorization to use airstrikes against the militant group over Syria. Britain joined the coalition flying over IS territory in Iraq in 2014, aiding Iraqi forces when they recently reclaimed the strategic city of Ramadi.

The video also showed the executions of five men accused of being British "spies."

British officials have said that some 800 British nationals are believed to have traveled to Iraq and Syria, many with the intention of joining the Islamic State militants.

ls/ (Reuters, AP)