There has been huge speculation over the reason for its disappearance, ranging from pirates to a mafia dispute to a commercial quarrel.
AFP news agency quoted Finnish police as saying a ransom demand had been made of the ship's Finnish owners, Solchart Management.
A source linked to the Cape Verde coastguard told AFP the Arctic Sea was outside its territorial waters.
The coastguard was informing maritime officials about the ship's movements, the source said, adding: "When the ship enters our jurisdiction, we will decide in consultation with our partners what actions to take."
Some reports have put the ship 400 nautical miles north of the Cape Verdean island of Sao Vicente.
French intelligence said it had found a ship matching the Arctic Sea's description in the area.
The Portuguese military would not confirm one of its planes had flown over the vessel.
Radar contact with the Arctic Sea was lost after it left the English Channel
However, the Russian ambassador to Cape Verde, Alexander Karpushin, said he had not been officially informed of the sighting and told Russia's RAI agency the sighting was "not true".
Tom Wilkerson, chief executive officer of the US Naval Institute, told the BBC the disappearance raised a number of concerns.
"What we're looking at is a ship that's over 4,000 tonnes, with no transponder working, that now all of the world's searching capability has not been able to find.
"Just because the ship doesn't appear to have anything on it of value doesn't mean that someone can't place something there that could be very valuable, and also very dangerous."
Last known contact
Carrying timber reportedly worth $1.8m (£1.1m), the Arctic Sea sailed from Finland and had been scheduled to dock in the Algerian port of Bejaia on 4 August.
It would seem that these acts, such as they have been reported, have nothing in common with 'traditional' acts of piracy or armed robbery at sea.
The crew reported being boarded by up to 10 armed men as the ship sailed through the Baltic Sea on 24 July, but the intruders were reported to have left the vessel on an inflatable boat after 12 hours.
There are also reports of the ship being attacked a second time off the Portuguese coast. However the ship's operators said they had no knowledge of the incident and Portugal said the ship was never in its territorial waters.
The last known contact with the crew was when the Arctic Sea reported to British maritime authorities as it passed through the Dover Strait.
On Friday, the European Union Commission spokesman Martin Selmayr said: "From information currently available it would seem that these acts, such as they have been reported, have nothing in common with 'traditional' acts of piracy or armed robbery at sea."
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