by Matt McGrath Environment correspondent, from BBC News
A large tanker carrying liquified natural gas (LNG) is set to become the first ship of its type to sail across the Arctic.
The carrier, Ob River, left Norway in November and has sailed north of Russia on its way to Japan.
The specially equipped tanker is due to arrive in early December and will shave 20 days off the regular journey.
The owners say that changing climate conditions and a volatile gas market make the Arctic transit profitable.
Built in 2007 with a strengthened hull, the Ob River can carry up to 150,000 cubic metres of gas. The tanker was loaded with LNG at Hammerfest in the north of Norway on 7 November and set sail across the Barents Sea. It has been accompanied by a Russian nuclear-powered icebreaker for much of its voyage.
The ship, with an international crew of 40, has been chartered from its Greek owners Dynagas by the Russian Gazprom energy giant. It says it has been preparing for the trip for over a year.