BEIRUT (Reuters) - A supporter of Islamic State militants has warned of attacks on the United States and its allies if they continue to carry out military action against the group that has seized large parts of Iraq and Syria, the SITE monitoring service said.

The message on a well-known militant Islamist online forum is one of the few responses from supporters of Islamic State to Washington's announcement last week that it was prepared to extend airstrikes against the group into Syria.

The posting on the Minbar Jihadi Media website condemned "intervention in the affairs of other peoples" and said it would trigger an equal response, SITE said late on Monday.

"It will lead to an equal reaction of the same strength in targeting the American depth and also the nations allied to it and in all aspects," the message said in a translation from Arabic, according to SITE, which tracks militant forums.

The United States has stepped up its military response to the hardline group, which has beheaded several Western hostages.

On Sunday Islamic State released a video that it said showed the beheading of a British aid worker.

U.S. President Barack Obama is calling for a coalition of Western and Middle Eastern countries to fight Islamic State.

"I direct a sternly worded warning to each of those nations involved with America, or that are allied with it in their war against the Islamic Caliphate, that their local and international interests will be legitimate targets," the posting by a supporter referred to as "Amir al-Thul" said.

The posting used religious language and said the author was speaking from a "blessed pulpit" but it was not clear what influence he had on the actions of Islamic State.

The message called on the public in the United States and its allies to oppose government actions against the group.

(Reporting by Sylvia Westall; Editing by Louise Ireland)