Estimates vary wildly. But Paul Beaver of the defence experts, Janes, speculates that Pakistan could have as many as 150 and India, 200-250.

India carried out underground nuclear tests at this site

The International Institute of Strategic Studies gives figures of 10-30 for Pakistan and 65-90 for India.

The majority of these warheads would be designed for delivery by bombs, not missiles.

But this is not a simple two-way race. China, estimated to have 300 to 600 nuclear warheads, is a prime factor driving India's weapons programmes.

And the fact that Pakistan's weapons programme has had considerable Chinese assistance has only increased India's concerns.

Missile race

The nuclear race in South Asia has spawned a parallel missile race as each country seeks to develop medium and long-range missiles capable of carrying a nuclear warhead.

Although India and Pakistan have said they do not intend to conduct further nuclear tests they carry out missile tests on a regular basis. India has already test fired its $8.5m Agni II missile. This has a range of 2,500km, meaning it could hit anywhere in Pakistan and worry China too.

It is now preparing to test its Agni III, reported to have a 3,500km range which could get it to Beijing.

India has also suggested it could make a neutron and a hydrogen bomb.

Pakistan has test launched its Shaheen I and Ghauri I and II missiles, the latter with a range of 2,500km. Work is in progress on a Ghauri III with a 3,000km range.

Test Ban Treaty

The 1998 nuclear tests were widely condemned by the international community and resulted in US sanctions.

But despite strong pressure from Washington, neither country has signed the Comprehensive Test Ban Treaty (CTBT), which bans nuclear test explosions.

India's Agni missile

Nor have they signed the Nuclear Non-Proliferation Treaty (NPT), which obliges the nuclear powers never to transfer their nuclear technology to other countries.

President Bill Clinton is desperately seeking a lever he can use to try to bring pressure to bear.

But some Indian critics say that if Washington really wants to stem the regional arms race, it should do more to discourage China's modernisation of its own nuclear arsenal.

Worsening relations

Relations between India and Pakistan, which have never been good, have deteriorated even further since last year's clashes in the disputed territory of Kashmir.

India Missiles

Tested

Agni I (1,500km) Agni II (2,000km)

Developing

Agni III (3,500km) Dhanush (250km ship launched)

The man India considers largely responsible for the latest conflict, General Pervez Musharraf, seized control in Pakistan in a military coup in October.

General Musharraf has said Pakistan will not try to match India in respect of the number of missiles produced.

Pakistan missiles

Tested

Ghauri I (1,300km) Ghauri II (2,000km) Shaheen I (750km)

Developing

Ghauri III (3,000km)

But he says it will retain just enough missile capacity ''to reach anywhere in India and destroy a few cities, if required''.

Defence spending

Last month India announced a massive 28.2% rise in defence spending, the largest single year increase in the country's history.

Prime Minister Atal Behari Vajpayee said India had ''to gear itself up" following the Kashmir conflict.

Defence analysts say this year's $13.94bn defence budget will be partly spent on building a credible nuclear deterrent.

India is also elaborating a new defence doctrine that gives its nuclear arsenal a clear deterrent role.

Pakistan is meanwhile setting up a high-powered authority to manage its nuclear arsenal, known as the National Command Authority.

The new body reduces concern about whether Pakistan's nuclear weapons are adequately controlled, but it also shows Islamabad is determined to develop its forces.

General Musharraf, who will play a key role in the authority, says Pakistan will follow a policy of restraint.

But he has warned it would consider using nuclear weapons if its security was threatened by India.