* Sees currency impact more than double previous forecast
* Vietnam, Europe strong, Nigeria, Russia weak
* Shares fall three percent (Updates after CEO interview)
By Philip Blenkinsop
BRUSSELS, Aug 1 (Reuters) - Heineken, the world's third-largest brewer, forecast a bigger blow from declining currencies this year after a first half when strength in Asia and Europe helped to offset declining sales in Africa.
The brewer of Heineken, Europe's best-selling lager, Tiger and Sol said its African markets were unlikely to recover in the near term and the weaker Mexican peso, Nigeria naira and British pound would reduce earnings.
The gap between Heineken and market leader Anheuser-Busch InBev will widen if AB InBev completes its 79 billion pound ($104.5 billion) takeover of SABMiller
Access to Africa is a major factor for AB InBev in the deal but Heineken's experience shows the road there can be bumpy.
Heineken Chief Executive Jean-Francois van Boxmeer told Reuters there would be no immediate pick-up in Africa and the Middle East.
"We still post growth because we are invested in other economies," said, "I speak particularly about Mexico or the whole of Asia as well as Europe which has been seen as a problem child for many, many years," he continued.
The company said Vietnam, Cambodia and Indonesia were the stand-outs in terms of growth, but it suffered in Russia, the Democratic Republic of Congo, Ethiopia, Egypt and major market Nigeria due to weak oil markets or political instability.
It expected its operating margin would expand by about 40 basis points over the year, slowing down following a 124 point expansion in the first half.
Heineken shares were down 3 percent at 81.91 euros at 0820 GMT, making them among the worst performers in the FTSEurofirst 300 index of leading European stocks.
Trevor Stirling, beverage analyst at Bernstein Research, described the guidance as "prudent", with margin erosion in the second half and the foreign exchange impact probably greater than the market had expected.
Heineken also forecast a much heavier hit from currencies, with a translational impact of 200 million euros ($223 million) at operating profit and 110 million at net profit level, more than double previous forecasts.
Weaker currencies include that of Heineken's largest market Mexico and the pound in Britain, where it is the market leader, as well as Nigeria's naira, which has abandoned its currency peg to the dollar and fallen by nearly 40 percent.
First-half operating profit before one-offs rose 12.6 percent on a like-for-like basis to 1.71 billion euros ($1.91 billion), above the average 1.67 billion euro expectation in a Reuters poll.
Net profit was up 11.2 percent at 977 million euros, below the average expectation of 998 million euros as the naira's devaluation was booked as a financial cost. ($1 = 0.8957 euros) ($1 = 0.7559 pounds) (Editing by Keith Weir)