The Islamic State group called on Muslims to kill citizens of all countries taking part in the US-led anti-jihadist coalition by any available means, in a statement posted online Monday.
"If you can kill a disbelieving American or European -- especially the spiteful and filthy French -- or an Australian, or a Canadian or any other disbeliever... including the citizens of the countries that entered into a coalition against the Islamic State, then rely upon Allah, and kill him," said Abu Mohammed al-Adnani, the group's spokesman.
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"Kill the disbeliever whether he is civilian or military," he said in the message, which was released in multiple languages.
The United States and France are carrying out air strikes against IS targets across Iraq and seeking to build an international coalition against a group increasingly perceived as a global threat.
US Secretary of State John Kerry said that more than 50 countries have come forward with commitments as part of the coalition, including Egypt and other Arab states.
The jihadist group, which has declared a "caliphate" straddling Iraq and Syria, controls swathes of territory in both countries.
IS is regarded as the most violent and powerful in modern jihad. It has executed hundreds of Iraqis and Syrians, as well as foreign hostages, and its brutal campaign has forced more than a million people from their homes.
Adnani's message -- which was released in an Arabic audio recording, together with transcripts in other languages including English -- gave instructions on how the killings could be carried out without military equipment.
"Smash his head with a rock, or slaughter him with a knife, or run him over with your car, or throw him down from a high place, or choke him, or poison him," the IS spokesman said.
"If you are unable to do so, then burn his home, car, or business. Or destroy his crops. If you are unable to do so, then spit in his face."
The hundreds of foreigners from various Western countries fighting alongside IS have sparked fears that they could return home to carry out attacks.
On Monday, Australian Prime Minister Tony Abbott warned that any of its citizens "who fight with terrorist groups... will be arrested, prosecuted and jailed for a very long time."
Adnani praised militants in Egypt's Sinai Peninsula, calling on them to "cut the throats" of those fighting for President Abdel Fattah al-Sisi.
"O America, O allies of America, and O crusaders, know that the matter is more dangerous than you have imagined and greater than you have envisioned," he said.
"We have warned you that today we are in a new era, an era where the (Islamic) State, its soldiers, and its sons are leaders, not slaves."
"We will conquer your Rome, break your crosses, and enslave your women, by the permission of Allah, the Exalted."
And he mocked the US air campaign, saying that "the battle cannot be decided from the air".
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"Are America and all its allies from amongst the crusaders and atheists unable to come down to the ground?"
Addressing Obama, Adnani said: "You claimed... that America would not be drawn to a war on the ground. No, it will be drawn and dragged. It will come down to the ground and it will be led to its death, grave, and destruction."