Leading article: Mr Obama's 'Af-Pak' options

This week's meeting in Washington of the presidents of the United States, Pakistan and Afghanistan marks an important stage in Barack Obama's pursuit of a so-called "Af-Pak" strategy. Unlike his predecessor, he sees Pakistan as a foreign policy priority and views its worsening turmoil as indivisible from the crisis in neighbouring Afghanistan, hence the diplomatic drive for the first summit involving all three leaders.

Mr Obama is right to stress the need for linkage between Afghanistan and Pakistan and right to press Hamid Karzai in Kabul and Asif Zardari in Islamabad to cooperate. Their regimes sink or swim together. Both are up against strong and growing Islamist insurgencies which neither they, nor the US, can afford to see gain much more ground if the momentum behind them is not to become unstoppable.

At the same time, both men are weak and unable to control their countries. Indeed, it is hard to see whose plight is worse, because while Mr Karzai long ago abandoned hope of establishing control over much of Afghanistan, Mr Zardari is now in the same boat – battling militants who have surged to within miles of the capital.

The dilemma for Washington is that while it has little confidence in the two presidents, it cannot abandon them, lest that destabilise the situation further. Meanwhile, the US is exploring options in Pakistan. Thus, while Mr Obama rolls out the red carpet for Mr Zardari, Hillary Clinton's team is reaching out to Mr Zardari's former ally-turned-rival Nawaz Sharif, seen as having more influence with the country's Islamists.

Whether the US can press Mr Sharif into a formal power-sharing deal is another matter. The ailing Mr Zardari might be grateful for any agreement that left him as president, while Washington believes that a broader political front in Islamabad might have more success in preventing a virtual Islamist takeover.

But the two men have a poor history of working together and Mr Sharif may well feel he has nothing to gain from propping up an unpopular president. Mr Obama may therefore find himself stuck with the two leaders he is greeting this week – diminishing his chances of improving security in this vital region.