Distribution and outsource specialist Bunzl reported a rise in interim pre-tax profits but warned that the challenging economic environment will continue to put pressure on underlying sales and profitability.Profit before tax and intangible amortisation was up 4% to £115.5m although down 11% at constant exchange rates. Broker Charles Stanley expected a slightly better improvement to £117m.Group revenue in the first half increased 17% to £2.29bn. However, at constant exchange rates revenue was marginally down due to the impact of more difficult macroeconomic conditions across the international markets in which the group operates.The interim dividend has been increased by 3% to 6.65p. 'The challenging economic environment will continue to put pressure on underlying sales and profitability although management initiatives implemented to reduce costs will help to mitigate the impact on profits,' said the group.'Although it is difficult to be precise about the future development of economies worldwide, the board believes that our market leading positions combined with the positive impact of currency, the results of cost reduction initiatives and our continued strong cash flow and balance sheet should enable the group to develop positively.'