The Syrian nation is dying as an indifferent world looks on, and the territory it occupies risks becoming "Afghanistan on the shores of the Mediterranean", the Turkish president, Abdullah Gul, has said.

Radicalisation of ordinary people by Islamist jihadist groups was spreading across Syria and posed a growing risk to its neighbours and the countries of Europe, Gul said in an exclusive interview with the Guardian.

But the response of the international community – including Turkey's American and British allies – to the security, humanitarian and moral challenges posed by the crisis had been very disappointing, he said. He reiterated his view that the UN security council's performance was a disgrace.

In a forthright and sometimes angry critique of western policy on Syria, Gul said the deaths of more than 100,000 people, mostly civilians, in fighting over the past 32 months could have been avoided. Turkish mediation efforts early on in the war were not supported and were even undermined by western powers, he complained.

With the conflict showing no sign of ending, Turkey faces extreme instability and dislocation along its 565-mile border with Syria, the prospective radicalisation of its Kurdish, Alevi and Sunni Muslim populations, the spread of infectious diseases such as polio, TB and measles, and many new additions to its current total of 500,000 Syrian refugees.

Asked about the risk of the war spreading beyond Syria, Gul said that if Turkey were attacked or Turkish territory invaded, his government would respond militarily "in the strongest way possible".

He said: "There is no question about this. In fact we have already stated that we have changed the rules of engagement and we have given authority to Turkish armed forces in that respect … I don't see how much worse it can get, it's already very bad. But let me also say that this is not a bilateral issue between Turkey and Syria. We did not have any conflict with Syria, but when those human rights violations begun to occur and there was massacring of the people of Syria, then it become a matter for mankind, for us all, the international community. It's only by virtue of being a neighbouring country that Turkey is so very much involved. Also from the point of view the fact that we are hosting 500,000 Syrians in Turkey. Two hundred thousand of them live in camps and 300,000 in the cities through their own means.

"We have so far spent $2bn (£1.25bn) for them. We will continue to do so because this is a humanitarian matter [but] it's very regrettable seeing the indifference on the part of the international community."

Gul said that if the current deterioration in Syria continued unchecked, it would pose an increasingly grave security and counterterrorism challenge for Turkey.

"If the atmosphere remains as it is, then this can lead to more radicalisation and some groups in the civil war becoming more extreme, dividing up, not being under control, and spreading across that country. Because under those circumstances, ordinary people could become much more extreme and this is something that poses a danger and threat not just for Turkey – it's an issue for everyone.

"I don't think anybody would tolerate the presence of something like Afghanistan on the shores of the Mediterranean. For that reason, the international community must have a very solid position with respect to Syria."

Unfortunately, that unified stance appeared lacking and meanwhile the country was being destroyed.

"At the very beginning the international community's rhetoric was very high [calling for president Bashar al-Assad to stand down immediately] but then it reverted to its current position now. And that is a contradiction itself. And morally too, there is a country, Syria, exhausting itself, consuming itself, with many people dead, the infrastructures gone, there is a lot impact of what is going on, and the international community is watching this. Simply watching this, and this is very regrettable."

Gul said Turkish efforts to engage Assad in a dialogue two years ago had not received sufficient backing from Turkey's allies, but could have headed off the disaster that ensued.

"We talked to Assad because we wanted things to be resolved by peaceful means. That engagement was at all levels, it wasn't just myself, the prime minister, the foreign minister, we all worked very hard and at the time we even faced pressure from our allies because they said this was going on too long and it wasn't going anywhere. This is what I mean about the high rhetoric of the international community at the very beginning. They should have done something to follow up on their rhetoric and this was not done.

"What we tried to do did not work out and there wasn't much more we could do. I wish Assad had understood what we were telling him. In my very last message to him I said that if things went on as they were, whatever might be done would be too little, too late, and that he should take the initiative and lead the change in his country so the country would not fall to pieces.

"He read my letter and said it was all very important and good but he did not act on it, he did not do anything. Most certainly, yes, if he had heeded my advice, 100,000 people may not have died and Syria would not have faced so much destruction."

Gul suggested that the chemical weapons deal with Assad brokered by Russia was a distraction that Assad had exploited to bolster his position.

"It might be said Assad made good use of an opportunity with the chemical weapons deal with the Russians. But the question comes back to the international community again … Of course we are very pleased about the chemical weapons and we support that. But was it just the chemical weapons? Do we reduce the whole thing to chemical weapons? I think there is a moral question there that is presented to the international community."

Asked whether the US and Britain should be doing more to end the crisis, he replied: "Our expectation was different, we expected more. I think it is very disappointing to see the whole discussion reduced to a discussion solely on chemical weapons."

Gul said it was important that the proposed Geneva II peace conference, due this month, was better prepared than its forerunner. He felt last month's London meeting of the Friends of Syria group had been helpful. But he expressed scant hope that the next Geneva meeting would achieve a breakthrough, if and when it takes places. "The country is destroyed … There really isn't in my opinion much that can be done now."

Gul spoke to the Guardian during a weekend visit to Edinburgh, where he attended the Turkey-Britain Tatli Dil bilateral forum, met Scottish residents of Turkish origin and was given dinner by the Duke of York.