WPP, the world's second largest advertising group, saw its shares dip 6% in early trading today despite posting strong revenue growth of 14.1% to £1.558bn for the first three months of this year.

The dip in WPP's share price, which was down 38p on its opening to 592p at 9.50am, has been attributed to disappointing figures for March, when the company's like-for-like organic revenue growth dropped to just 3% year on year.

Across the first three months of the year, WPP's organic revenue growth excluding the impact of acquisitions and currency volatility, was 5%.

However, WPP chief executive Sir Martin Sorrell admitted that like-for-like revenue growth was 6% in both January and February and just 3% in March.

WPP's US business remained buoyant across the first quarter of 2008, but an unexpected slowdown hit its western European operation.

"Whilst January and February were strong across the board, both geographically and functionally, and indeed stronger than last year, March was slower, somewhat surprisingly in western continental Europe," said WPP in a statement.

North America proved to be the surprise star performer for WPP, despite the uncertainty gripping the US economy, with like-for-like revenues up by more than 5% year on year to £580.9m – an improvement over the second half of 2007.

"The only explanation is that packaged goods make up 25% of our business [in North America] and that most companies, if not all, have increased prices to cover commodity price increases and this has led to more advertising to promote that," said Sorrell.

"The US remains generally stronger, except for some sectors such as financial services, in the first quarter than the second half of last year."

Eastern continental Europe – now the fastest growing region globally for WPP – and the Middle East both posted like-for-like revenue increases of more than 21%.

"Western continental Europe showed some softening in March, particularly in the major markets of Germany, France and Spain," said WPP

"The United Kingdom showed some improvement with growth of almost 2%, a small increase over the last half of 2007."

Sorrell said he was not concerned about WPP's growth dip in March, adding that "nothing has gone negative, North America has, in fact, surprisingly gone the other way".

WPP said it remained on target to achieve its full-year margin objective of 15.5% and that global revenues were "in line with budget".

The company brought in new advertising business worth £564m in the first quarter.

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