* Sept. deliveries helped by extra selling day -analysts
* Sales up only 1.3 pct if adjusted for selling days -analyst
* Q4 sales may slow on strong year-ago period -analyst (Adds comments from analysts and background)
By Andreas Cremer
BERLIN, Oct 17 (Reuters) - European car sales rose 6.1 percent year on year in September, benefiting from an extra selling day compared with 2013 and with brands like Volkswagen and Ford enjoying stronger sales in Germany, France and Italy.
Europe's car market bottomed out last year, the end of a six-year slump, and has posted growth for 13 straight months. But the recovery is still threatened by weak consumer confidence and geopolitical uncertainties in Russia and elsewhere and analysts cautioned that growth could slow in the fourth quarter.
Adjusted for the extra selling day this year, European deliveries were up a more modest 1.3 percent compared with September 2013, and may decline slightly in the fourth quarter, compared with a strong year-ago period, said Arndt Ellinghorst, a London-based analyst with ISI Group.
"We urge investors to react with caution," Ellinghorst said. "The situation is still difficult, there are only gentle impulses" driving auto market growth.
In August, growth was 1.8 percent year on year.
September's new passenger-car registrations in the European Union (EU) and the countries of the European Free Trade Area (EFTA) totalled 1.269 million vehicles, up from 1.196 million a year ago, the Association of European Carmakers (ACEA) said.
The International Monetary Fund last week trimmed its global economic outlook for this year and next. The euro-area economy stalled in the second quarter with Germany and Italy contracting and France stagnating.
Still, ACEA data showed those three countries rebounded in September after contracting a month earlier.
Sales in Germany, the region's biggest market and home of VW, Daimler and BMW, gained 5.2 percent to 260,062 models. Third-placed France was up 6.3 percent and No. 4 Italy gained 3.3 percent.
Overall, nine-month European sales rose 5.8 percent to 9.91 million vehicles from 9.36 million in the year-earlier period, according to ACEA.
Momentum is expected to slow in the remainder of the year and the possible return to recession in the euro zone poses a risk for demand in 2015, said Jonathon Poskitt, head of European forecasting for LMC Automotive.
LMC earlier this month trimmed its full-year forecast for sales gains in Western Europe to 4.6 percent or 12.7 million vehicles. Volume growth may slow further in 2015 to 3.3 percent or 12.5 million, Poskitt said.
Sales in Spain, Portugal and Greece, formerly hit by austerity measures, were up nearly 30 percent or more, stoking demand for VW's passenger-car brand, Ford and General Motors' Opel division. Deliveries of the three marques grew 7.5 percent, 6.5 percent and 6.2 percent respectively, according to ACEA.
Luxury nameplates Audi and Mercedes-Benz, both locked into an intense sales battle with premium champion BMW, rebounded from declines in August, delivering 2.9 percent and 9 percent more cars, respectively. BMW, by contrast, saw September deliveries fall 2.1 percent on top of a 2 percent drop a month earlier.
PSA Peugeot Citroen, recovering from a sales slump that saw it lose billions of euros and rely on French government bailouts, posted almost 10 percent higher sales, extending year-to-date deliveries by 4.7 percent. (Reporting by Andreas Cremer; Editing by Biju Dwarakanath and Elaine Hardcastle)