The price of benchmark Brent North Sea crude oil for delivery in October lost 11 cents to 40.35 dollars per barrel in early deals.

New York's reference contract, light sweet crude for October delivery, tumbled 1.03 cents to 42.96 dollars per barrel in pre-opening electronic trading. The US market had been closed on Monday for Labor Day.

"An absence of fresh bullish news is causing the market to slide a little bit," said Prudential Bache broker Christopher Bellew.

Recent comments from the Opec producers group predicting a fall in prices as the cartel raises output might have been partly behind the decline, he said.

Ministers of the Organization of Petroleum Exporting Countries are due to meet at their Vienna headquarters next week to discuss output policy.

Concerns about the threat from tropical storm Frances in the Gulf of Mexico eased, though traders said disruption to import activities in the region could have an effect on weekly US oil inventory figures due out on Thursday.

"Oil prices have weakened over-night after a storm threat to US crude oil production in the Mexican Gulf subsided and the market interpreted price cuts announced (Monday) by Saudi Arabia to its European crude oil sales, as a sign of a weaker fundamental picture for oil," Barclays Capital analysts wrote in a note to clients.

Hurricane Frances, which had been downgraded to a tropical depression, was located about 30 miles (48 kilometres) southwest of Albany, Georgia, moving towards the north-northwest, according to the National Hurricane Center in Miami.

The hurricane "probably caused some precautionary close-downs (of US oil installations), and that may well translate into lower stocks again in the statistics when they come out a day late on Thursday", Bellew said.

He said oil prices were likely to rebound soon, either in response to the oil inventory figures or before then in the event of any supportive news.