New York's main contract, light sweet crude for September delivery, fell six cents to 78.92 dollars a barrel, while Brent North Sea crude, also for September, was eight cents weaker at 77.42 dollars.
"There are a lot of mixed signals," said David Johnson, a Hong Kong-based oil analyst with the Royal Bank of Scotland.
"There is not enough confidence in the US and Europe recovery.... A lot of people are getting slightly concerned about what's happening," he told AFP.
Stock markets mostly shrugged off on Monday the results of Europe's banking stress tests, which gave approvals to all but seven European lenders, prompting scepticism among many experts.
Providing a lift was the latest data from the US Commerce Department showing that new home sales had increased 23.6 percent to 330,000 units in June, from a revised May rate of 267,000. But again sceptics pointed out that the rise was off rock-bottom lows.
Investors are now awaiting a weekly government report on US energy stockpiles, regarded as a gauge of demand in the world's biggest economy.
Last week's inventory data showed an unexpectedly big rise in crude stocks and investors expect the report to show a drop in reserves when the Department of Energy releases its report Wednesday.
