Despite having quelled riots in the cities, the Yemeni government faces a continuing - and potentially more serious - challenge from tribes on the desert fringes.

Relations between the state and tribes in the north-eastern border provinces of Ma'rib and al-Jawf have always been fragile, and since steep price increases were announced in June at the behest of the IMF, security forces in the area have clashed with the Jahm, Jida'an and Alaiwah tribes.

In the past, tribes with a grievance have often resorted to kidnapping foreigners, especially in Ma'rib. But now they have latched on to another, more permanent hostage: the pipeline that carries almost 40% of Yemen's oil production to Ra's 'Isa on the Red Sea.

On July 10 they blew another hole in the pipeline near Sirwah - the seventh since the trouble began. So far, both the government and the Hunt Oil Company which operates the pipeline, have played down the damage. After an earlier attack, a company spokesman referred to small leaks caused by bullet holes and said: "The volume of oil which has leaked is too small to be measured accurately."

However, recent reports have put the spills at more than 20,000 barrels and, according to the Yemen Times, the total cost of repairs will exceed $1 million. As MEI went to press, one spill had been burning for nine days because the area was thought too insecure for an attempt to douse the flames and repair the leak.

Although the 350-km pipeline is vital to the country's economy and important (via Hunt Oil) to Yemeni-American relations, it is virtually impossible to protect. If the damage is indeed relatively slight, this is probably only because the tribes have so far attacked it with guns and grenades rather than more powerful explosives.

With the vulnerability of the pipeline - as well as its political and economic significance - exposed, the government faces a long-term problem in its relations with the tribes whose territory the oil passes through. President Salih's initial response has been to place a more ruthless army commander in the area, though historically the only effective way to deal with the tribes has not been to control them but to reach an accommodation. At present the omens for that are not good either: last week Sheikh Mohammed al-Zaidi of the Jahm tribe returned to Ma'rib after the failure of negotiations in Sana'a.

Yemeni officials have blamed both the Saudis and the opposition Islah party for fomenting trouble in Ma'rib and in the cities. Both have an obvious motive: the long-running border dispute in the case of the Saudis, and the publicly-declared aim of bringing down the government in the case of Islah. The Saudi ambassador in Yemen categorically denied that the kingdom had "aided or instigated" the disturbances, though a number of northern tribal sheikhs are widely believed to be Saudi clients.

Whatever the truth, the Yemeni government habitually blames external forces for internal problems. In this instance, the tribes do have a number of specific demands, including compensation for flood damage three years ago, plus a reversal of the price increases.

In the southern city of Aden there have been at least two explosions within a week near the oil refinery, apparently caused by grenades. Eighteen people, including fishermen, were taken in for questioning after the first incident, which a government spokesman said had caused no damage "worth mentioning". However, a local newspaper claimed that a fuel pipe feeding ships in the harbour had been damaged, causing extensive pollution.

Meanwhile the official SABA news agency announced last weekend that Yemen had finalised a 40-year soft loan of $60 million from the World Bank. It will be used to build a new power station for Sana'a and upgrade existing equipment - which should alleviate the capital's frequent electricity cuts.