Russia has accused Senegal of engaging in "piracy" after armed naval officers from the west African nation seized 62 of its nationals on a trawler for alleged illegal fishing.
The Russian foreign ministry on Thursday called in the Senegalese charge d'affaires for a 40-minute meeting during which it filed a formal protest aimed at signalling the seriousness with which Moscow treated the affair.
Russian diplomats made "a firm demand to the Senegalese authorities to immediately release the Russian fishing trawler, the Oleg Naydenov, and its crew," foreign ministry spokesman Alexander Lukashevich said.
He said Senegal's charge d'affaires had assured him that Dakar would "make every possible effort to resolve the situation".
Russia's federal fisheries agency argued on Thursday that the ship was actually in the waters of Senegal's southern neighbour Guinea Bissau when it was seized.
"You can say we are talking about piracy on a state level," Alexander Savelyev, a spokesman of Russian fisheries agency, told Rossiya 24 state television.
He said the ship was now suffering daily losses of more than $30,000 and promised to raise the matter with the International Tribunal for the Law of the Sea.
"We have been preparing the required documents for the past two days," Savelyev was quoted as saying by Russia's Prime business news agency.
The same Hamburg-based UN tribunal had earlier ordered Moscow to release the Greenpeace crew Russian authorities had detained in November - a hearing boycotted by the Kremlin.
Savelyev further accused Senegalese authorities of acting under the pressure of Greenpeace activists who -- angered at their earlier arrest - were urging Dakar to take a tough stance with the Russian crew.
"It turn out that the army of the sovereign Republic of Senegal is acting under Greenpeace orders," Russian media quoted the fisheries spokesman as saying.
Senegal said on Sunday that it planned to fine Russia about $800,000. The ship and its crew - which also includes 23 citizens of Guinea Bissau - are being held under armed guard in Dakar.
The nation of 14 million has for years been battling to contain unauthorised fishing in its waters by foreign trawlers.
It blames this activity for depleting its stocks of sea life and endangering the livelihoods of local small-scale fishermen.
Senegalese Fisheries Minister Haidar El Ali said on Thursday that at least three illegal Russian trawlers had been caught off its waters in the past few days.
The spokesman for a union of Senegalese fishing industry workers added that Senegal suffered from "aggressive interventions by ships from Russia, Ukraine and Belize".
"These boats are repeat offenders," Senegalese fishing union spokesman Adama Lam told reporters in Dakar.
Senegal detained the Oleg Naydenov vessel in the Atlantic on January 4 after accusing it of repeatedly fishing in its waters without a proper permit.
The incident has created a furore in Russia's state media amid anger at what Moscow views as discrimination against its nationals by a small African state.
The trawler's captain has complained of being refused the right to see a doctor and a Moscow delegation in Dakar has accused Senegalese officials of denying them meetings or explanations about the case.
Fishing authorities in Moscow have also linked the ship's arrest to a clandestine campaign by Greenpeace - an environmental protection group whose activists were detained for two months last year after staging a protest against oil drilling off Russia's Arctic coast.