Tony Blair has called for more efforts to break down segregation in Northern Ireland and stressed the continued need to be “vigilant” about the threat from terrorism.

The former prime minister, who brokered the Good Friday agreement in 1998, said there was no longer community support for terrorists in Northern Ireland but added “you can’t drop your guard at all”.

Blair said the deal 18 years ago was only a beginning and that major barriers of segregation needed breaking down.

He told BBC Radio 4’s Today programme on Friday: “Some of the deep social problems and their political consequences still remain and what is true also is that it is still very easy for politics to break back into sectarianism.

“The answer to this is that it takes a long period of time. I think in Northern Ireland we have got to be realistic, we have also got to keep working at it, because we didn’t stop working at it and I think you have got to do that if you really want the reconciliation to take hold.”

The attack on prison officer Adrian Ismay, who died after a dissident republican bombing, and the threat of violence marking the centenary of the 1916 Easter Rising showed the continued danger posed by terrorism, Blair said. However, he insisted there had been a change in the attitudes of communities in Northern Ireland.

“You have got to be apprehensive and you have got to be vigilant for sure. But the big difference is that when the IRA were operating, and some of the loyalist groups were operating when we came into office in 1997 and for the decades before that, they were operating often with significant support within local communities.

“I don’t think that support for this terrorism is there in the local communities today. But there will be some on the fringes that want to engage in violence and therefore you can’t drop your guard at all.”

Blair also said progress could be made on tackling segregation in Northern Ireland’s politics. “You have got to break down segregation. It’s not done always by laws or political agreements, it is also done at a grassroots level, it is done by changes of attitude in the mindset over time and it’s also by achieving a sense in the politics that people want to move on,” he said.

“You have still got a situation where the parties in Northern Ireland, they have a power-sharing agreement but they are still very much defined by their position from traditional communities. In time you have got to think about how you break those barriers down as well, for sure, but this takes a long time. It’s not going to be easy.”

He added: “You know when a democracy has taken root and that is when you elect the best person in your judgment to lead the country. It’s not where the person came from or what background, it’s what they stand for and what they can do for the people as a whole. It is not surprising we are not at that point yet in Northern Ireland but we can get there.”

Asked about the possibility of Sinn Féin taking power both north and south of the border, Blair refused to be drawn: “The one thing I do remember from my time in Northern Ireland is there are certain questions you answer and certain questions you don’t. That’s definitely in the category of one you don’t.”