Britain must be prepared to join a wide coalition of countries in deploying ground troops, including special forces, to combat forces from the Islamic State (Isis), the former prime minister Tony Blair has said. In a lengthy essay on the threat posed by Isis on the eve of the UN general assembly meeting in New York, Blair warned that air strikes alone would not be enough to combat the jihadis.

The intervention by Blair comes as Britain considers whether to join the US and France in launching air strikes against Isis forces in Iraq and possibly in Syria. On Wednesday, David Cameron will attend a meeting of the UN Security Council in New York that is to be chaired by Barack Obama.

The prime minister may use the UN meeting to advocate British involvement in the air campaign, though he has ruled out sending in ground troops. But Blair made clear that air strikes were insufficient to confront Isis.

The former prime minister, now Middle East peace envoy, wrote on his website: "Air power is a major component of this, to be sure, especially with the new weapons available to us. But – and this is the hard truth – air power alone will not suffice. They can be hemmed in, harried and to a degree contained by air power. But they can't be defeated by it."

Blair stressed that he was not calling for a repeat of the lengthy military campaigns in Iraq and Afghanistan. He wrote: "We're not talking here about armies of occupation. We are, in certain situations where it is necessary and subject to all proper limitations, talking about committing ground forces, especially those with special capabilities."

In his article, Blair also warned that western governments are in danger of making a "fateful error" in assuming that they should challenge only Islamist extremists who advocate violence. Echoing the warnings by Michael Gove, who has called for a "draining of the swamp", Blair called for action against the fringe of extremists who advocate violence as well as the wider spectrum that believes in religious exclusivity.

He wrote: "This Islamism – a politicisation of religion to an intense and all-encompassing degree – is not confined to a fringe. It is an ideology (and a theology derived from Salafist thinking) taught and preached every day to millions, actually to tens of millions, in some mosques, certain madrassas, and in formal and informal education systems the world over.

"It is the spectrum that helps create the fringe. A large part of western policy – and something I remember so well fighting in government – is based on the belief that we can compromise with the spectrum in the hope of marginalising the fringe. This is a fateful error. All we do is to legitimise the spectrum, which then gives ideological oxygen to the fringe."

Blair called on western leaders to avoid being naive as he said that unreformed Islamism is incompatible with modern economies. He wrote: "You cannot uproot this extremism unless you go to where it originates and fight it. The spectrum is a different matter. Here the most important thing is to expose it, to speak out against it, to make sure that at each point along the spectrum the proponents of this ideology are taken on and countered; but also be prepared to engage in dialogue and to acknowledge, as has been the case in Tunisia, that some of those on this spectrum may be willing to leave it. So there should be openness in our attitude, but the total absence of naivety. To engage successfully, we have to be willing to confront.

"We are not doing this as of yet. The truth is that Islamism, unless fundamentally reformed, is incompatible with modern economies and open-minded, religiously pluralistic societies. This truth has to be recognised."