Royal Dutch Shell has been given preliminary permission to start drilling in the Arctic, raising hopes that it will be able to complete its two planned exploration wells in the region before the threat of encroaching sea ice makes operations impossible.

Ken Salazar, the US interior secretary, said Shell would be allowed to begin work on a well in the Chukchi Sea off the north-west coast of Alaska, as long as it did not drill into any oil-bearing rocks.

The decision, marking the culmination of more than seven years of preparation for Shell, is a significant step forward for a project that offers the potential for discovering huge oil resources but also carries high costs and environmental risks.

Pete Slaiby, the vice-president of Shell Alaska, described the move as “extremely exciting”.

However, he warned that it would still be “very, very difficult” to complete the well in the Chukchi Sea unless Shell was granted an extension on the deadline for stopping drilling set by the government.