David Cameron has urged the United Nations to stand up to regimes that persecute their own people, in a strong defence of liberal interventionism.

In his first speech to the General Assembly since taking office, the prime minister said the UN had to seize the opportunity presented by the Arab Spring to become more than just a talking shop.

"The UN has to show we can be not just united in condemnation but united in action," he said."The UN is no more effective than the nation states that come together to enforce its will."

He added: "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?"

Cameron's speech appeared to display a new found fondness for interventionism following the seemingly successful use of force against Colonel Gaddafi.

The prime minister said that the UN's response to the crisis in Libya had demonstrated it had "the will to act" and in doing so stopped Benghazi from joining Srebrenica and Rwanda "in history's painful roll call of massacres that the world failed to prevent".

"The international community found its voice in Libya, and must not now lose its nerve," he said.

He called for a "credible resolution" threatening "tough sanctions" on Syria in response to the bloody crack down on anti-regime protesters there.

"To fail to act is to fail those who need our help," he said.

But the prime minister also warned that the West had to be careful to be seen to impose its own values and versions of democracy on others.

"The mistake we often make in the West is that because people want democracy they want it in the same way and with the same outcomes that we do," he said.

And he returned to a theme he has previously explored when he said that democracy was "a process not an event" and that participatory government requires "much more than the simple act of voting".

Cameron could also not resist taking an off-script swipe at Iranian president Mahmoud Ahmadinejad.

He said Ahmadinejad "runs a country where they have elections, of a sort, but suppress freedom of speech" and "detain and torture those who argue for a better future".

Ahmadinejad addressed the assembly earlier in the evening and repeated his suggestion that the September 11 attacks were a conspiracy.

The claim led some Western delegations, including those from the United States, France and other European nations, to walk out.

On Wednesday, Cameron met Barack Obama at the UN as Palestinians prepared to bid for statehood on Friday. It has been reported that the British government is divided on how to respond to the move, with deputy prime minister Nick Clegg keen on greater recognition for the Palestinians.