Around 20 Greenpeace activists blocked access to a shale gas exploration site in Romania on Monday, in a third attempt to scupper the drilling operations of US energy group Chevron.

Protesters locked hands and sat in front of the entrance to a drilling position in the northeastern village of Pungesti.

They prevented a lorry from entering and displayed banners reading "No to fracking".

Fracking is a controversial extraction technique that consists of injecting water and chemicals deep into rock to release gas.

Environmentalists warn that chemical-laced waste could contaminate fresh water resources, while the fracking itself could cause minor earthquakes.

Chevron started drilling its first exploratory Romanian well in May despite fierce opposition from residents who fear environmental damage in an area dependent on agriculture.

"Shale gas exploitation can not be done if it affects the life of hundreds of thousands of people living in the areas given in concession", said Laurentiu Ciocirlan, the Greenpeace campaign coordinator.

Chevron insists that its operations respect security standards and says that a decision on possible full-scale exploitation is to be made only in three to five years.

Later on Monday, a Chevron statement said the company respected "the right of individuals to express their opinions; however it should be done within the law".

Previous attempts to drill in Romania were suspended twice at the end of 2013 due to demonstrations by shale gas opponents.

Chevron argues that shale gas could prove crucial for Europe's energy security, especially as a crisis deepens in neighbouring Ukraine, a key transit country for Russian natural gas supplies.

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