A week before Pakistan's first nuclear tests in May 1998, then prime minister Nawaz Sharif received a late night telephone call from a Saudi prince. India, Pakistan's arch- rival, had conducted nuclear tests that month and Mr Sharif was weighing the consequences of following suit.
As Mr Sharif told a hurriedly organised meeting of senior officials, the Saudi prince had offered up to 50,000 barrels of oil a day to Pakistan for an indefinite period on deferred payment terms. This would allow Pakistan to overcome the impact of punitive western sanctions expected after the tests.
The Saudi message, delivered on behalf of Crown Prince Abdullah, the de facto ruler, once again bailed out Pakistan at a difficult moment.
"It is possible that Pakistan may still have conducted its nuclear tests without the Saudi oil. But the tests would have been done with the knowledge that the economic fallout was going to be far more severe," says a former aide to Mr Sharif.
Saudi financial support has fuelled suspicions of nuclear co-operation between the two countries.
A senior US official says Saudi finance helped fund Pakistan's nuclear programme, allowing it among other things to buy nuclear technology from China.
Officials discount the possibility of Pakistani help to build an indigenous Saudi nuclear weapon. But they say there could be a sort of "lend-lease arrangement" that would allow weapons from Pakistan to be made available to Saudi Arabia. "The argument that they have options on Pakistan's arsenal is more likely," the US official says.
Both Saudi and Pakistani officials vehemently deny the existence of any such deal. "We've never given money aimed at nuclear research and development and so we never asked or received privileges to nuclear weapons programmes," insists Prince Turki al-Faisal, the former Saudi intelligence chief.
Nawaf Obeid, a Saudi security consultant close to the government, however, suggests the kingdom enjoys Pakistan's security umbrella without any formal agreement. "We gave money and they dealt with it as they saw fit," he says of the Pakistanis. "There's no documentation but there is an implicit understanding that on everything, in particular on security and military issues, Pakistan would be there for Saudi Arabia."
The relationship has been thrown into sharp focus recently with the uncovering of a clandestine nuclear network led by disgraced Pakistani scientist Abdul Qadeer Khan. This sent investigators in search of the so-called "fourth customer" beyond the three to which Mr Khan confessed supplying: Libya, Iran and North Korea.
Diplomats close to the Vienna-based International Atomic Energy Agency say Mr Khan tried to find customers all over the Middle East but they have yet to find evidence to implicate a fourth country. Nor is there any suggestion so far that Saudi Arabia purchased nuclear equipment or expertise from the Khan network.
To be sure, Saudi Arabia has plenty of reasons - and the financial muscle - to seek nuclear weapons. Saudis live in a dangerous environment, surrounded by rivals. They include Israel, whose undeclared nuclear arsenal Saudi Arabia criticises as the main block to a nuclear-free Middle East, and Iran, Saudi Arabia's strategic competitor suspected by western governments of developing nuclear weapons.
In the 1980s, when Saddam Hussein was considered a close friend of Saudi Arabia, Iraq's military strength was seen as protection for the Sunni Muslim monarchies of the Gulf against the ambitions of a revolutionary Shia regime in Iran.
After Mr Hussein invaded Kuwait in 1990, however, Iraq became the main threat in the Gulf and the Saudis called on the US for protection. Relations between Iran and Saudi Arabia have meanwhile gradually improved over the past decade, though they remain beset by suspicion.
Nearly all US troops stationed in the country since the Gulf war were withdrawn last year following the removal of Mr Hussein's regime, leaving a few advisory and support units. Political ties with the US also became strained in the backlash from the September 11 attacks, carried out by mostly Saudi militants.
"Saudi Arabia is in strategic limbo, with the US security commitment being called into question or being redefined and with Iran's nuclear programme," says Wyn Bowen, a lecturer in war studies at King's College, London.
Against this troubled background, the link with Pakistan has become all the more important. "It's probably one of the closest relationships in the world between any two countries without any official treaty," says Prince Turki, now ambassador to London.
Reports of Saudi nuclear ambitions have been around since the 1970s, fuelled partly in the late 1980s by the purchase of 30 or more nuclear-capable, medium-range Chinese missiles. Though both countries said they had been adapted to carry conventional weapons, US pressure following the purchase led Saudi Arabia to sign the Nuclear Non-proliferation Treaty.
Saudi officials say the country's leaders always considered that nuclear weapons would bring the kingdom more controversy than comfort. Last year, in the aftermath of the Iraq war, senior princes considered a paper that offered three options: acquiring a nuclear capability as a deterrent, allying with a nuclear power that would offer protection, or working to rid the region of banned weapons. Prince Turki insists the paper "died in its place".
Robert Einhorn, a senior arms control official in the Clinton administration, acknowledges there is little hard evidence that Saudi Arabia is pursuing the bomb: "It's like a suspected crime where you have a motive but not much more than that."
Over the years, however, Saudi Arabia's discreet but deep ties with Pakistan have kept suspicions of nuclear co-operation alive.
Rooted in co-operation between military generals and intelligence operatives, the relationship survived repeated political upheavals in Pakistan.
The two countries also have been drawn together by religious ties: the Saudis, custodian of Islam's two holiest sites, have been eager to protect a country, also governed by Sunni Muslims, that came into existence because of its religion. Moreover, the kingdom has also poured money into religious schools - madrassas - spreading its puritanical brand of Wahabi Islam throughout Pakistan.
Saudi officials say Pakistan has probably received more Saudi aid than any other non- Arab country. In return, the Saudis received military and diplomatic assistance.
In the 1960s, Pakistani instructors were dispatched to Saudi Arabia to train Saudis on the use of newly acquired British aircraft. In the 1970s, an agreement was reached with Pakistan to second 15,000 military personnel to the kingdom. They pulled out in 1987, at a time of depressed Saudi oil revenues.
In the 1980s and 1990s the two countries found common cause in arming the Arab fighters who helped drive the Soviet Union from Afghanistan. After the Soviet withdrawal and Afghanistan's descent into civil war, both favoured the Taliban militia which emerged from the Wahabi religious schools in Pakistan.
Hasan Askari Rizvi, a leading Pakistani analyst on defence and national affairs, says Saudi Arabia paid for 40 F-16 fighter aircraft bought by Pakistan in the 1980s from the US for $1bn.
"Not only did the Saudis pay for
the aircraft but they also lobbied for Pakistan with the US government,"
he says. "The Saudis have played a critical role for Pakistan. That has
won them tremendous influence in Islamabad."