A salvage team is expected to spend the night on a grounded oil rig on Lewis after finding a safe access in difficult weather conditions.

The Transocean Winner rig was carrying 280 tonnes of diesel - more than 300,000 litres - when it was blown ashore in severe weather conditions on the western side of the Isle of Lewis early on Monday.

The rig is believed to have leaked more than 50,000 litres of fuel, most of which is thought to have evaporated.

Read more: Exposed: the safety record of stranded oil rig company​

The semi-submersible installation detached from its tug the Alp Forward during towing from Norway to Malta and the weather prevented the line being reconnected.

Bad weather has meant the salvage team has only briefly been able to board the vessel for checks but teams were able to use sea ropes to partially climb the structure on Saturday.

Rope access technicians have now created a safe alternative access after the prevailing weather conditions made it too difficult to winch the team down to the installation.

A team of eight salvors will assess the rig and work to make it habitable so that they can remain on it overnight.

There is also work being done to help provide a supply line for the team so they can get equipment and supplies on board.

The Maritime and Coastguard Agency said: "The team from Transocean and Smit will look at the diesel oil tanks, looking at how they might start procedures to transfer any fuel to other unbreached tanks within the rig so that it will be in a safe location well above the waterline, before any operation to refloat the rig commences.

"There's also work going on in close proximity to the Transocean Winner to carry out a multi-beam survey looking to identify the safest route to move it when the time comes.

"An additional aircraft is going to be brought in later on Sunday to help with the work.

"An exclusion zone of 300 metres remains in place around the rig covering the sea and the air, which means no drones will be permitted in the area, providing a safe working area for the salvage team to work in."

Read more: Diesel spill feared after oil rig runs aground on Isle of Lewis​

Hugh Shaw, the Secretary of State's representative for maritime salvage and intervention, said: "Once the assessment is under way, we'll have a much better idea of what we are dealing with, which will mean a more detailed salvage plan can be drawn up and put into place.

"We've made a commitment to keeping people informed locally and we intend to keep to that - once I'm happy that the plan is ready, we will be sharing it with community leaders and the community as a whole."

Last week Mr Shaw said the ''worst-case scenario'' is that about 52 tonnes, more than 50,000 litres, of oil has leaked into the environment.

However, no oil has been seen on the surface of the sea and Mr Shaw said most of the escaped diesel would have evaporated.

A ''low level'' of pollution was detected by tests on Monday, Western Isles Council said, but daily shoreline inspections by the Scottish Environment Protection Agency have indicated no visible sign of pollution.

No-one was on board the rig when it grounded at Dalmore beach near Carloway.

People are being urged to stay away from the beach as teams continue to work and a temporary 300-metre (980ft) exclusion zone has been set up at sea.

Read more: Rig incident tug moored in island port​

Environmental groups have raised concerns while it is expected to take ''some time'' to refloat the rig as it came ashore at high tide with a storm surge.

Scotland's Environment Secretary Roseanna Cunningham said serious questions need to be asked as to why the rig was being towed during severe weather and Deputy First Minister John Swinney had raised the point with the UK Government.