A government minister in Mali also said officials had "no confirmation" that the two Spaniards had been released.
"We have no confirmation that the two hostages have been freed as I am speaking to you," the minister told AFP news agency.
Earlier this month a Malian official had said a branch of al-Qaeda in the Islamic Maghreb (AQIM), which has executed two hostages, was threatening the lives of the two Spanish nationals, who are held by another branch of AQIM.
But the NGO the men worked for said at the time the families had seen proof that the two were still alive and that negotiations for their release were continuing.
They were among three Accio Solidaria workers kidnapped in Mauritania last November and handed over to AQIM, in exchange for payment.
AQIM released the third hostage - Alicia Gamez - in March.