Canberra - Australia and Britain are on heightened terror alert even as Saudi Arabia's king warned that the West will be the next target of the militants sweeping through Syria and Iraq unless there is "rapid" action.
"If we ignore them, I am sure they will reach Europe in a month and America in another month," King Abdullah of Saudi Arabia said at a welcoming ceremony on Friday for new ambassadors, including a new envoy from the United States.
He added: "Terrorism knows no border and its danger could affect several countries outside the Middle East. I ask you to transmit this message to your leaders: 'Fight terrorism with force, reason and (necessary) speed'."
Fighters with the extremist group, the Islamic State in Iraq and Syria (ISIS), have prompted widespread concern as they advanced in both Syria and Iraq, killing hundreds of people, including in gruesome beheadings and mass executions.
Governments around the world are also increasingly concerned that their citizens who have joined the group may return with plans to attack on their home soil.
On Friday, Britain raised its terror threat to "severe", the second-highest level, due to new intelligence. This means an attack is "highly likely" but that there's no intelligence about a specific plot. The highest level is "imminent".
The British public could expect to see more police, including armed officers, on the streets as a result of the increased threat level, Prime Minister David Cameron told reporters in London.
He will also announce new legislation tomorrow to make it easier to confiscate the passports of people suspected of wanting to travel to join extremist groups.
Australia raised similar concerns yesterday, with Attorney-General George Brandis saying: "It is of serious concern that Australian citizens are active in these groups."
Australia's terrorism public alert system, however, remained at "medium", indicating an attack could occur, he added. Canberra has assessed that there are about 60 Australians fighting in Syria and Iraq for terrorism groups, with about 100 more involved in facilitation and support for those groups.
White House press secretary Josh Earnest said there are no plans to raise the threat level in the United States. President Barack Obama earlier this month authorised air strikes against ISIS militants in Iraq while ordering surveillance flights over Syria.
Yesterday, the six-nation Gulf Cooperation Council strongly condemned the ISIS and its extreme interpretation of Islam as it opened a meeting in Saudi Arabia.
It also said it supported a United Nations Security Council resolution earlier this month that calls "on all member states to take national measures to suppress the flow of foreign terrorist fighters", and threatens sanctions against anyone involved in their recruitment.
Ahead of a Nato summit in Wales, US Secretary of State John Kerry has called for a global coalition to combat the ISIS' "genocidal agenda".
Writing in The New York Times on Friday, he said in his op-ed article that the US would put forward an action plan at a summit meeting of the UN Security Council next month.