Former British Prime Minister Tony Blair has warned of terrorist attacks larger than those carried out in Paris in which 130 people were killed if the threat of the Islamic State (Isis) was not seriously addressed.

Speaking at an event at the US Library of Congress on Thursday (3 December), Blair laid out what he said was a comprehensive strategy for defeating IS (Daesh), which he said must begin with a recognition of the size of the challenge after the Paris attacks.

"A continued failure to recognise the scale of the challenge and to construct the means necessary to meet it will result in terrorist attacks potentially worse than those in Paris, producing a backlash which then stigmatise the majority of decent law-abiding Muslims and puts the very alliance that is so necessary at risk creating a cycle of chaos and violence," said Blair.

In October Blair acknowledged that the invasion of Iraq played a part in the rise of IS, and apologised for some mistakes in planning the war. In Washington DC on Thursday Blair said that IS must be fought militarily and ideologically.

"But destroying the so-called caliphate is an essential part of destroying the concept of it as a unifying structure for extremist groups which underpins so much of the jihadist propaganda. It is also essential to securing a just outcome in Syria which in turn of course is the only solution to the refugee crisis," he said.

Critics say the US decision to disband Saddam Hussein's army after the 2003 invasion created a huge security vacuum exploited by al-Qaeda, which was eventually replaced by Islamic State.