Washington, Sep 24 (IANS/EFE): US security forces are on alert against the possibility of an attack by "lone wolves" in reaction to the US bombings on the Islamic State (IS) Sunni radical group in Syria, media reported.

A circular from the FBI and the Department of Homeland Security Tuesday warned that extremists living in the country could decide to carry out an attack on national soil in the face of US military intervention in predominantly Muslim lands.

The text, published by the media in the US, was sent Tuesday night to all security agencies of the country to increase surveillance in order to abort any terrorist plot.

The alert comes after the US began bombings in Syria against IS positions and the Khorasan cell, a lesser known group of Al Qaeda veterans which, according to the Pentagon, was preparing an "imminent" attack against Western interests in the US or Europe.

The FBI circular said, however, that the bombings in Syria might have interrupted temporarily any plan of attack (by Khorasan or the IS).

For US authorities, the greatest threat is not an attack by these groups but rather the ability of the jihadis to inspire "lone wolves" into action in the nation.

A month ago, the FBI and the Department of Homeland Security sent a similar circular alerting against possible attacks in retaliation to the US offensive launched Aug 8 on the IS in Iraq.

On Sep 18, New York and Las Vegas tightened security in main tourist attractions after the IS issued a call on the internet for "lone wolves" to carry out attacks in these cities with homemade bombs.

The recruitment of Western citizens to fight alongside the IS is one of the major concerns of the US and European governments.

A Yemenite with American citizenship, Mufid Elfgeeh, was recently accused of recruiting militants to fight in the ranks of the IS in Syria and Iraq and attack Shia Muslims and the military on US soil.