Aug 3 (Reuters) - The International Air Transport (IATA) said on Thursday it expects a record breaking summer travel season for the northern hemisphere after global carriers reported a 12-year high in traffic growth for the first half of 2017.

Global demand for air travel rose 7.8 percent in June as all regions reported growth, thanks to brighter economies and lower air fares, IATA said. In the first six months, demand was up 7.9 percent, while load factors - a measure of how full planes are - reached a first-half record of 80.7 percent.

Europe's airlines have been benefitting from the demand, with many increasing their profit targets for the year and reporting strong first-half figures over the last few weeks.

"But as costs rise, this stimulus of lower fares is likely to fade. And uncertainties such as Brexit need to be watched carefully," IATA head Alexandre de Juniac said in a statement, referring to Britain's upcoming exit from the European Union.

Overall, June capacity measured in available seat kilometres rose 6.5 percent, slower than demand, meaning load factors increased 1 percentage points to 81.9 percent, IATA said in its monthly traffic update.

(Reporting by Silvia Recchimuzzi in Gdynia; Editing by Victoria Bryan) ((Gdynia.newsroom@thomsonreuters.com; +48 58 772 0920))