UK Prime Minister David Cameron said today he was ready to order air strikes on Islamist militant targets in Libya and Syria to prevent attacks on the streets of Britain as he stepped up his rhetoric against Islamic State insurgents.

He was speaking to reporters as he landed in Indonesia on the first leg of a four-day trade mission he hopes to use to forge new political alliances to counter a threat he has described as a "death cult".

"I've said with respect to Libya, or indeed with respect to anywhere else, if there is a plot under way where I believe British citizens are in danger of being targeted, if it's possible to take action to stop that, I would. That is legal, that is right, that is proper and that's the role of the prime minister," said Cameron.

Britain raised its domestic terrorism alert to the second-highest level in August last year, saying an attack was "highly likely".

Cameron met Indonesian President Joko Widodo later today to discuss how the two countries could cooperate in the battle against Islamist militancy.

The Islamist threat is high on the political agenda in Britain after a gunman killed 30 British tourists at a Tunisian beach resort last month in an attack claimed by Islamic State.