The warning of a combined terror attack was sounded by National Security Guards(NSG) Director General J N Choudhury, who also called the 2008 Mumbai carnage that killed 166 people as just a "curtain raiser".
The NSG at the same time said it was fully prepared and will respond to whatever required and whenever required.
The NSG warning came a day after a top Army officer said that instances of waving of ISIS flags in Kashmir was a matter of concern and "deserves the highest attention of the security agencies" so that the youth in the Valley are prevented from being lured into the ranks of the jihadist organisation.
"Now that they (international terror outfits like al-Qaeda) have declared their intention of attacking India, they will obviously align with known groups that we have (active in the country). Whether it is Laskhar (Lashkar-e-Toiba), Jaish (Jaish-e-Mohammad), Harkat (Harkat-ul-Mujahideen) or IM (Indian Mujahideen).
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"So the apprehension we have is that if they do have a combined kind of strategy or combined operations, we have to prepared and be alert if a combined (terrorist) group takes action. Then there is a threat and possibility of a multi-city multiple attack. That is what we are preparing for and hence we are also working with anti-terrorist forces and state police forces (in this context)," the head of the 'black cat' commandos told reporters here.
Choudhury said he remembers from his earlier stint in the Intelligence Bureau that al-Qaeda operatives had done a "reconnisance of Goa, Bangalore and Amritsar" about a decade ago.
"This (recce of vulnerable spots in India) is not a new thing for al-Qaeda," the NSG chief said, adding they looked at Goa as it was visited by a number of foreign and domestic tourists.
Lt Gen Subrata Saha, General Officer Commanding of the Army's Srinagar-based 15 Corps, while noting that the ability of the Islamic State in Iraq and Syria (ISIS) to attract a large number of volunteers was a cause of worry, said that concerted efforts have to be made to secure the "right to prosperity" of the youth in the Valley if they were to be kept away from the path of terror.
"The ability of ISIS to attract large number of volunteers is a matter of concern. There are believed to be 10,000 to 15,000 volunteers at present fighting for ISIS. The fanaticism shown by the fighters is also a cause," he added.