By Mike Thomson BBC Radio 4's Today programme correspondent in Pyongyang

North Korea is a jittery place. Every morning and evening, the sound of air raid sirens fills the air and big voices bark instructions from roof-top speakers.

They tell the people of Pyongyang to run to bomb-proof shelters which are usually to be found in the city's deep underground system.

CRISIS CHRONOLOGY

16 Oct: US announces that N Korea has acknowledged secret nuclear programme 14 Nov: Oil shipments to N Korea halted 22 Dec: N Korea removes monitoring devices at Yongbyon nuclear plant 31 Dec: UN nuclear inspectors forced to leave North Korea 10 Jan: N Korea pulls out of anti-nuclear treaty 4 Feb: US says it might reinforce troops in Pacific 5 Feb: N Korea says nuclear facilities reactivated

Timeline of tensions

At night, blackout drills take over from regular power cuts in ensuring the curtains are drawn and lights are out.

The city is not at war and no bombs are falling, but the feeling here is that it is only a matter of time.

The enemy is not here either, but that does not stop him being hated and feared.

Giant posters cover the city's billboards.

Courageous North Korean soldiers stare out from their hoardings urging local citizens to join the "sacred battle to annihilate enemies".

Everybody knows who they are. They do not need to be named.

The first casualty in this whipped-up war of hyperbole is the American dollar.

It was dropped from its spot of favoured hard currency at the end of last year and replaced by, of all things, the euro.

Pre-emptive strike

But going red over America's military muscle is a far trickier problem.

Pyongyang has watched with growing alarm the bolstering of US forces around the Korean peninsula.

Washington says the extra aircraft carriers and bomber planes are to replace forces that have gone to Iraq, but the North Koreans are not convinced.

In fact, fiery words are now coming from offices in the city's foreign ministry. Deputy Director Kim Pyong-gap left me in no doubt of his government's displeasure.

"If the US stepped their boots over the borderline, we will take strong counter-measures," he said

In being asked what that means, Mr Kim replied: "A pre-emptive attack is not only something the US can do. We can do it too. It's a matter of life or death."

America, along with the rest of the world, will be hoping this is a bluff. After all, such a first strike on the most powerful country in the world would be something akin to suicide.

It would not be a rational act. Yet everybody now knows that North Korea does not necessarily have a rational government.

It is going to be a very tough call.