By Mark Davies BBC News Online political reporter in Islamabad

Pakistan tested a third ballistic missile just hours after Jack Straw arrived in the country in a bid to prevent a war with India over the disputed Kashmir region.

The foreign secretary, who touched down in Islamabad in the early hours of Tuesday, is meeting Pakistan President General Pervez Musharraf and Foreign Minister Abdul Sattar before flying on to New Delhi.

Mr Straw is expected to urge General Musharraf to do more to crack down on militants as a first step towards easing the grave crisis.

I don't know if we can prevent war, but I know we have to try

Jack Straw, Foreign Secretary

But hours before the meeting, Pakistan tested its Abdali warhead, which has a range of 180 kilometres (110 miles) and would be able to reach most border positions in India.

And the Ministry of Defence has confirmed it will be following up a request from India about the possible sale of the Sea Harrier.

Delhi already has a �1bn order with British Aerospace for Hawk trainer jets.

Nuclear fears

Speaking on the plane before landing in Islamabad, the foreign secretary told the BBC: "The world is watching the situation in India and Pakistan with great concern.

"I don't know if we can prevent war, but I know we have to try."

Mr Straw warned again on Monday of the real prospect of the region spiralling towards a nuclear war.

The foreign secretary said the world would be "astonished and bewildered" if such a war broke out between India and Pakistan.

Defeating terrorism

He said such a conflict would have grave consequences, with "death, destruction, disease and economic collapse" affecting many parts of the subcontinent for years to come.

Mr Straw's starting point with the Pakistani leader will be to urge action against terrorism.

If there are positive signs of that, he believes the next stage is to persuade both sides to drastically reduce the military build-up along the line of control in Kashmir.

Only then does he believe talks between India and Pakistan could resume.

Mr Straw admits that the international community is facing a tough task in heading off the crisis, but adds: "Because we cannot do everything, that does not mean we should not do anything."

Straw's optimism

The foreign secretary, who met Pakistan's foreign minister ahead of the meeting with the president, went into his meetings optimistic that in the long term the diplomatic effort will bear fruit.

By making Islamabad the first stop on his visit, he is signalling that the onus is on Pakistan to take the first steps towards easing the crisis.

Mr Straw is visiting Islamabad and New Delhi

Mr Straw will then visit Delhi as part of the international community's growing efforts to persuade India to step back from the threats of war.

There is a recognition in the British camp that some steps have been taken by both sides, but the message is that more needs to be done.

And there are fears that opinion in India is such that President Musharaff would have to go a long way to convince Delhi that he will follow words with action.

There is also concern that those in India who favour an increasingly tough stance are calling the shots.