A vessel which broke down with several hundred illegal migrants on board is being refused permission by Mauritania to dock or allow passengers ashore.
The ship is thought to be carrying mostly Pakistani migrants.
It was reached by a Spanish vessel on Saturday and towed to within 12km of Nouadhibou fishing port.
Mauritanian Red Crescent and Spanish Red Cross officials said a boat with urgently needed supplies of food and water had been sent to the vessel.
"Mauritania has nothing to do with this boat breaking down, nor with taking charge of those on board,” said a foreign ministry statement on Monday.
Neighbouring Senegal has also said it could not handle the ship.
The Spanish foreign ministry earlier reported that Mauritania had agreed to help with the rescue.
There were initially thought to be around 200 people, mostly Pakistani, on board the vessel.
An official from the Mauritanian Red Crescent in Nouadhibou, Ahmedou Ould Haye, told Spanish RNE radio there could be as many as 500 migrants.
He told the French news agency AFP that the migrants included “200 Asians from Kashmir” as well as Africans.
But he did not say if they were from Pakistan or Indian-administered Kashmir.
The Mauritanian foreign ministry had earlier said there were 200 Pakistani nationals on board.
Senegal and Mauritania are launching points for tens of thousands of African migrants looking to reach Europe, many via the Canary Islands.
About 30,000 Africans were caught trying to reach the Spanish island chain last year.
EU patrols are now trying to stem the flow of immigrants.