Opec could review its output limits this week

Workers in Nigeria are threatening a three-day strike from 11 April in protest at the use of casual labour.

Claims by Saudi Arabia that it could boost supply by 1.5 million extra barrels of oil a day had little impact on oil prices.

US sweet, light crude reached $57.02 a barrel, a rise of 0.5%. In London, Brent crude was steady at $55.59.

Opec move?

Last week, the Opec cartel of oil-producing nations announced plans to increase production by about 500,000 barrels a day in order to cool prices.

Opec may meet this week to discuss further output increases.

The threat of a strike in Nigeria has however pushed prices higher.

Nigeria is the ninth-largest producer of oil, with an output of about 2.5 million barrels a day.

US demand for oil rises as the summer holidays approach

Oil workers in Nigeria are giving energy companies and the government a 21-day ultimatum to stop using casual staff.

A strike could seriously disrupt the supply of oil and drive prices even higher, though it is by no means certain to take place.

Two-year stint

Earlier on Monday, Saudi's oil minister Ali al-Naimi said the country had spare capacity of 1.5 million barrels of oil a day that it could use if necessary.

"We can go up a million and a half barrels per day," the Saudi oil minister Ali al-Naimi told reporters in Manila earlier on Monday, according to Reuters.

He added: "It depends on demand, all we need is customers."

Opec president Sheikh Ahmad al-Fahd al-Sabah "has the authority to suggest uplifting the ceiling to 28 million bpd (barrels per day)," Mr Naimi said.

On Saturday, the International Monetary Fund (IMF) warned that oil prices could stay at high levels for the next two years because of high levels of demand and limited supply.

However, IMF managing director Rodrigo de Rato said prices would not necessarily stay at about $56 a barrel.

"We have to be aware that probably oil prices will stay high, although probably not at these levels, in the next two years at least because of demand pressures and because of certain supply constraints."

Mr de Rato predicted global economic growth of above 4% despite the IMF's expectation of high oil prices.