Concerns may be mounting over the likely impact of the current Wall Street turmoil on US cleantech investment, but according to new figures the sector continued to attract record levels of interest from investors during the second quarter of the year.

New research from analysts Topline Strategy Group showed that after "disappointing" quarters at the end of 2007 and beginning of 2008, the sector rebounded in the second quarter of the year with a record 66 deals and $897m invested.

The surge in investor interest broke the previous record of 55 deals and $736m set during the third quarter of 2007.

The report said that the increased venture capital investment was driven by the maturation of investments made over the past two years, noting that while first round funding levels remained steady, "most of the growth in Q2 was fuelled by follow on rounds from earlier investments".

The findings provide further evidence that cleantech projects ranging from wind and solar farms to electric cars are now entering a scale up phase where investor risk is diminishing but larger sums are required to build up production capacity.

However, experts remain concerned that it is these types of larger scale projects that could be hardest hit by the tightening credit conditions that have emerged in the wake of the collapse of Lehman Brothers and take over Merrill Lynch.

The report also confirmed that cleantech hubs are fast appearing across the US, predicting that "cleantech portfolio companies will be nearly as geographically concentrated as high tech and life sciences".

It added that Silicon Valley, New England, New York Metro, Los Angeles and the Pacific Northwest were now the top five locations for cleantech venture backing.