Oil and gas giant Shell has been granted the final permit it needs to begin drilling below the ocean floor for oil in the Arctic.

Shell began work last month, but was allowed to drill just the top sections of two wells, off the coast of Alaska.

Environmental campaigners are against the drilling which they say could harm the region.

But the US government said it was monitoring Shell's work "around the clock" to ensure the "utmost safety".

"Activities conducted offshore Alaska are being held to the highest safety, environmental protection, and emergency response standards,'' added Brian Salerno, director of The Bureau of Safety and Environmental Enforcement (BSEE), which issued the final permit.

A Shell spokesperson also emphasised the firm's focus on safety. "We remain committed to operating in a safe, environmentally responsible manner and look forward to evaluating what could potentially become a national energy resource base," the spokesperson said.

The final permit - granted after the firm repaired a key piece of equipment - is an extension to one issued in July, and will allow Shell to drill into oil-bearing rock below the ocean floor in the region for the first time since its last exploratory well was drilled over two decades ago.

The company has already committed about $7bn (£4.5bn) to the controversial project, and is confident it will find huge quantities of oil in the region.

Experts believe that more than 20% of the world's undiscovered oil and gas resources can be found in the Arctic.

Environmentalists argue vehemently that the oil should be left well alone, as the risks of damaging this pristine environment are too great.

But Shell believes that despite the environmental risks, oil can be extracted safely - oil that it argues will be needed to meet burgeoning demand for energy across the world over the coming decades.