Submarines accompanying a Russian naval mission aimed at boosting Moscow's claim to Arctic territory are shortly expected to dive below the North Pole.
The expedition set sail from Murmansk port on the Barents Sea
The two Mir submarines plan to dive to the ocean floor, 4,200m (14,000ft) below the pole, to carry out tests and plant a capsule with a Russian flag.
A Russian official said the "risky and heroic" mission was comparable to "putting a flag on the Moon".
Melting polar ice has led to competing claims over access to Arctic resources.
Russia's claim to a vast swathe of territory in the Arctic, thought to contain oil, gas and mineral reserves, has been challenged by other powers, including the US.
The Russian expedition set off last week from the port of Murmansk and is looking for geological evidence to back up Moscow's claims to the resource-rich Arctic seabed.
Russian media reported last week that the ships were briefly tailed by foreign aircraft, but this claim was played down by the expedition leader.
The expedition is being led by two members of parliament - Arthur Chilingarov, a seasoned polar explorer, and fellow MP Vladimir Gruzdev.
The Itar-Tass news agency reported on Wednesday that the expedition's two ships - a nuclear-powered ice-breaker and a research vessel - had arrived at the North Pole.
The two mini-submarines are shortly expected to leave the ships and begin their dive to ocean floor.
Scientists aboard the submarines plan to collect samples of Arctic flora and fauna and leave behind a titanium capsule containing a Russian flag.
The submarines' return from the seabed to the surface is regarded as the most dangerous part of the journey.
The vessels will have to navigate back to the exact point where they started from, or else risk being trapped beneath the Arctic ice.
"This is a risky and heroic mission," Sergei Balyasnikov, a spokesman for Russia's Arctic and Antarctic Institute, told the RIA-Novosti news agency.
"It's a very important move for Russia to demonstrate its potential in the Arctic," he said. "It's like putting a flag on the Moon."
President Vladimir Putin has already described the urgent need for Russia to secure its "strategic, economic, scientific and defence interests" in the Arctic.
Moscow argued before a UN commission in 2001 that waters off its northern coast were in fact an extension of its maritime territory.
Then and now: Nasa photo shows ice cover in September 2005 and as it was in September 1979
The claim was based on the argument that an underwater feature, known as the Lomonosov Ridge, was an extension of its continental territory, but it was rejected and Russia told to resubmit with more evidence.
Several countries with territories bordering the Arctic - including Russia, the US, Canada and Denmark - have launched competing claims to the region.
The competition has intensified as melting polar ice caps have opened up the possibility of new shipping routes in the region.
Current laws grant countries an economic zone of 200 nautical miles beyond their land borders.
This zone can be extended where a country can prove that the structure of the continental shelf is similar to the geological structure within its territory.
The North Pole is not currently regarded as part of any single country's territory and is therefore administered by the International Seabed Authority.
RUSSIA'S ARCTIC CLAIM
1) North Pole: Russia plans to leave its flag on the seabed, 4km beneath the surface, as part of its claims for oil and gas reserves
2) Lomonosov Ridge: Russia argues that this underwater feature is an extension of its continental territory and is looking for evidence
3) 200-mile line: Shows how far countries' agreed economic area extends beyond their coastline. Often set from outlying islands
4) Russian claimed territory: The bid to claim a vast area is being closely watched by other countries. Some could follow suit