IT IS funny what people choose to worry about. The West is obsessed with stopping Iran getting nuclear weapons. By contrast, Pakistan's nuclear programme is not much discussed. And yet, by any sensible measure, Pakistani nukes are much more worrying.

Start with the obvious: Pakistan already has nuclear weapons - probably more than 100 of them - and is thought to be increasing production. Iran has still to assemble a single nuclear weapon. The prospect of an Iranian bomb is said to be unthinkably dangerous because of the country's connections to terrorist groups, its hostility to the West and Israel, the risk it will spread nuclear technology and the prospect of a regional arms race. And yet, almost all these considerations apply even more forcibly to Pakistan.

Pakistan supplied nuclear technology to North Korea, Libya and Iran. It came dangerously close to nuclear conflict with India in 1999. As for terrorism, Osama bin Laden was actually living on Pakistani soil for many years and the tribal areas of Pakistan are still al-Qaeda's most important base.

Pakistan was also the launch pad for the terrorist attacks in Mumbai in 2008, in which 164 people were killed. Although Pakistan's government condemned the attacks, there is strong evidence that the terrorists had links to Pakistani intelligence.

If the Mumbai attacks had been launched from Iran, the West would be shouting about "state-sponsored terrorism". With Pakistan, all you get is awkward mumbling.

Of course, there are reasons for this difference in treatment. Unlike Iran, Pakistan is nominally an ally of the US and receives billions of dollars in aid. Ashfaq Kayani, the chief of staff of the Pakistani military, is a charming fellow who once studied at Fort Leavenworth in the US. As senior Pakistanis are swift to point out, many of their soldiers have died fighting Islamist militants.

But Pakistan has yet to come up with a satisfactory explanation for the fact that Bin Laden was living just a stone's throw from a big Pakistani military academy. The Pakistani reaction to the raid that killed Bin Laden was one of anti-American outrage rather than self-criticism. A doctor who helped the US track down Bin Laden has just been sentenced to decades in prison in Pakistan.

In the aftermath of the Bin Laden raid, many in Pakistan speculate that the US may be planning another raid to seize the country's nuclear deterrent. Partly in response to that, Pakistan is believed to have cranked up production of nuclear weapons and fissile material and to have adopted a policy of moving its nukes around more frequently. The threat of a nuclear weapon "falling into the wrong hands" is obvious. Just as worrying is the rise of Islamist militancy within the ranks of the Pakistani military itself.

While visitors to Iran often report that the general public is well-disposed towards the US, no visitor to Pakistan can miss the country's deep anti-Americanism. Episodes such as the Bin Laden raid and the repeated US drone strikes on militants in Pakistan have plunged relations between the US and Pakistan to a new low. About 69% of Pakistanis say they regard the US as an enemy.

Yet it is Iran's non-existent nukes that continue to obsess the West. Diplomats have spent so long trying to stop Iran that I get the impression they no longer even ask themselves why it is such a high priority. Press them, and you will get explanations about the dangers of a Middle Eastern arms race and Iran's regional ambitions.

Interestingly, few seem to take seriously the idea that Israel often evokes - that Iran might actually commit nuclear genocide.

Western concerns are valid. But, in themselves, they do not seem compelling enough to explain the desperate focus on Iran. The main reason the Iranian dossier is so urgent seems to be the fear that Israel will soon attack Iran's nuclear facilities, provoking a wider war. US and European diplomats are reluctant to put it quite that directly, since this carries the uncomfortable implication that western policy is driven by Israel. But when people say "time is running out" over Iran, it is the prospect of an Israeli attack they are usually thinking about.

Most of those I know, in government and outside, who have a close knowledge of the Iranian nuclear issue seem to believe that Israel is likely to attack this summer. Last week, I thought I had found a dissenter. But he simply said: "Israel will wait until September or October because the weather is better and it's closer to the US elections."

For Israel, it does make sense to worry more about Iran than Pakistan. Iran has missiles that could hit Israel. Pakistan's missiles do not have the range; its nuclear doctrine is focused on India. But the terrorists based in Pakistan are no friends of the Jewish state. One of the targets they attacked in Mumbai was a Jewish cultural centre.

In the end, the desperate effort to stop the Iranian nuclear programme - while living with Pakistani nukes - may have a simple explanation. Pakistan already has nuclear weapons. Iran can still be stopped.

But next time somebody tells you that Iranian nuclear weapons would be an unparalleled and intolerable threat to international security, you might remember that we are already living with a more alarming menace: the Pakistani bomb. ©2012 The Financial Times Limited.