THE United Nations should be prepared to intervene more often against rogue states, David Cameron insisted in his first speech to the organisation's General Assembly in New York.

In a hastily inserted passage, the Prime Minister also launched a broadside against the Iranian President Mahmoud Ahmadinejad, who used his earlier speech to denounce America, Israel and Europe.

A reference to the “mysterious 9/11 incident” prompted a mass walkout by delegates.

Mr Cameron said: “He didn’t remind us he runs a country where they may have elections of a sort but they also repress freedom of speech, do everything they can to avoid the accountability of a free media, violently prevent demonstrations and detain and torture those who argue for a better future.”

In a strong defence of lawful interventionism, the PM said the Arab Spring and conflict in Libya had demonstrated the UN needed “a new way of working”.

He told world leaders the wave of uprisings represented a “massive opportunity to spread peace, prosperity, democracy, and, vitally, security, but only if we really seize it”.

Mr Cameron said the bid for a more democratic future across North Africa and the Middle East presented a challenge to the region and beyond.

The UK leader, who has won plaudits for his swift action in leading the campaign against Colonel Gaddafi’s regime in Libya, asked: “You can sign every human rights declaration in the world but if you stand by and watch people being slaughtered in their own country when you could act, then what are those signatures really worth?”

He argued the UN had to show that it could be “not just united in condemnation but united in action”.

The Prime Minister praised the bravery of the Libyan people, declaring the revolution in their country truly belonged to them.

He pointed out the coalition of nations across the West and the Arab world had prevented Benghazi becoming another Srebrenica on history’s painful roll call of massacres.

While likening the movement for democracy in the Maghreb to the fall of the Berlin Wall and the end of apartheid in South Africa, Mr Cameron warned the West should not try to impose its values on the region.

However, he set out just how daunting the challenges were for the region, 60% of whose population was under 25 but where youth unemployment was nearly double the world average.

This meant it had to create 50 million new jobs by 2020 simply to keep pace with its population -- 700,000 new jobs each year in Egypt alone.

The PM stressed how these jobs should not just be for men but women too -- the jobless rate for women in Egypt was more than three times that of men.

He argued that, now the voice of the region was finally being heard, there was a “unique opportunity for women to fulfil their ambitions too”.

Mr Cameron claimed the international community had found its voice in Libya but warned it must not now lose its nerve, referring to Syrian and Yemen.

“We must have the confidence to speak out and act as necessary to support those who seek new freedoms,” he declared.

The Prime Minister said he recognised many nations had been committed to non-intervention but added: “My argument is that where action is necessary, legal and right, to fail to act is to fail those who need our help.”

Last night, Sir Menzies Campbell, the former Liberal Democrat leader, cautioned about adopting a policy of interventionism, saying while action in Libya had the support of the Arab League and the Africa Union plus the legal authority of a UN resolution, “in other circumstances, it may be difficult to create a similar favourable environment”.