LONDON  - Crude oil prices swung Monday back towards last week's record highs as oil workers in Brazil began a five-day strike that further tightened global energy supplies, traders said. New York's main oil contract, light sweet crude for August delivery, added 82 cents to 145.90 dollars a barrel. The contract had hit a peak of 147.27 dollars on the New York Mercantile Exchange on Friday. London's Brent North Sea oil for August gained 61 cents to 145.10 dollars, after hitting an all-time high of 147.50 dollars last Friday. "Prices are firm ... as the risk of supply disruptions around the world looms large," Barclays Capital analysts wrote in a note to clients on Monday. In Brazil, workers in the country's main oil-producing region began a five-strike on Monday, cutting overall daily output by nearly a third, their union said. Thirteen of the 42 oil platforms owned by the state-run company Petrobras in the affected Campinas basin off the coast from Rio de Janeiro were virtually shut down, the United Oil Workers Federation (FUP) said in a statement. As a result, Brazil's output had been cut to 1.4 million barrels per day, instead of the normal 1.8 million barrels, FUP said. Traders said mounting geopolitical tension in the oil-rich Middle East and unrest in key producer Nigeria also provided underlying support for prices. "It seems clear that geopolitics will continue to provide fuel for the seemingly relentless rally, with Iran and Western nations still at a standoff over the Islamic republic's pursuit of a uranium enrichment programme," said Linda Rafield, senior analyst at energy information provider Platts. Tehran insists its nuclear drive is aimed solely at generating energy but some Western nations fear it could be aimed at making an atomic bomb and have called for a freeze of its uranium enrichment programme. Iran is the second biggest crude producer in the OPEC cartel. The country's output is about four million barrels per day. OPEC has said the cartel would not be able to replace Iran's oil production if supplies were halted in case of a war with Israel or the US. Exacerbating supply concerns is the ongoing unrest in Nigeria, where violence in the southern delta region has reduced the country's total oil production by a quarter since January 2006. Earlier on Monday, oil prices fell as investors followed the stronger dollar and took profits from last week's record-breaking surge. The strengthening US unit tends to dampen oil prices because it discourages demand for dollar-priced crude which becomes more expensive for buyers using weaker currencies. The dollar won support after Washington announced measures to bolster Fannie Mae and Freddie Mac, two giant firms holding almost half of all US mortgage debt which were pushed to the brink by the US subprime home loan crisis. In a separate development for the oil market, US President George W. Bush was to announce later Monday that he is lifting an executive ban on oil drilling on the US outer continental shelf and urged lawmakers to follow suit. "The president will announce that he has decided to lift the executive ban on oil exploration in America's outer continental shelf" in a public statement at 1:30 pm (1730 GMT), White House spokeswoman Dana Perino told reporters.