International Consolidated Airlines Group, more widely known as IAG, said on Tuesday that group traffic increased by an impressive 10.6 per cent year-on-year in December, helped by strong growth at British Airways.The airline group said revenue passenger kilometres totalled 15,025m last month, up from 13,591m in December 2012.The increase, however, was only 3.6% on a pro-forma basis, which includes results from Spanish carrier Vueling in the prior year's figures. IAG officially took control of the airline in April 2013.Nevertheless, December's results showed that traffic growth had accelerated from November, when revenue passenger kilometres rose at an annual rate of 8.4%, up 1.6% on a pro-forma basis.The addition of Vueling helped IAG to carry 5.23m passengers during December, up 29.2% from the year before. Otherwise, the number of passengers carried increased by 4.3% on last year on a pro-forma basis.Meanwhile, traffic at British Airways rose by 6.9% last month, making up for continuing tough conditions at its Iberia arm, where traffic fell 11.4%.The stock extended gains following the statement on Tuesday, trading up 3.1% at 426.8p by 15:29.BC