Saudi Arabia's King Abdullah is warning the West that extremists could launch their next attacks inside the United States and Europe, if there is no stronger international response to the terrorist activities being perpetrated by Islamic State militants in Iraq and Syria. 

The Saudi king spoke at a reception for foreign ambassadors in Riyadh Friday night.

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"If neglected, I am certain that after a month they will reach Europe and, after another month, America,"  King Abdullah said, in a statement that was also carried by official Saudi Arabian media.

Abdullah did not mention what group could launch these attacks on U.S. and European soil, but it expressed his frustration at what is perceived to be weak responses by the west to the atrocities being perpetrated by Islamic State militants.

Two days earlier, Islamic State militants released a video showing Syrian government soldiers being forced to march across the desert in their underwear. Later videos showed rows of soldiers lying dead in the sand, while photos showed IS militants shooting soldiers with automatic kalahnikov rifles at the back of the head while they kneel.

The executions were believed to have happened after IS militants reportedly overran a Syrian airbase a few days ago.

The IS has also publicly declared they have executed members of minority groups in northern Iraq after taking control of several towns in the area inhabitted by Christian and Yazidi minorities.

"These terrorists do not know the name of humanity and you have witnessed them severing heads and giving them to children to walk with in the street," King Abdullah said, while urging the ambassadors to relay his message directly to their heads of state. 

Saudi Arabia has been openly supporting Sunni rebels fighting against Syrian president Bashar Assad, but denies directly funding or arming the Islamic State militants.

Earlier this month, the country's top cleric described the Islamic State group and al-Qaida as Islam's No. 1 enemy, saying that Muslims were their first victims. The kingdom has now threatened to imprison its citizens who fight in Syria and Iraq in support of the Islamic State group.