Moody's Investors Service is lowering its rating outlook for the 17-country region that uses the euro currency, as uncertainty over the European debt crisis grows and the stronger countries in the group could be hard-pressed to provide support.

The rating agency said Monday that it is revising its outlook to "Negative" from "Stable" for the European Union's top Aaa credit rating. That means the rating could be downgraded, which would cost the EU more to borrow. Moody's said the move reflects the currently "Negative" outlooks for the Aaa ratings of Germany, France, Britain and the Netherlands, which together contribute about 45 percent of the EU's budget.

Those countries likely would put a higher priority on repaying their own debts than helping cover EU debts, Moody's said Monday.