French suspect Anis Bahri was arrested at the flat in the southern port city on March 27 at France's request. He is suspected of planning an attack in France for the Islamic State group along with Reda Kriket, another suspect arrested near Paris a few days before him.
"We have found around 45 kilos of ammunition, including two types of bullets which can both be used with Kalashnikovs," Wim de Bruin, a spokesman for the national prosecutor's office said.
He said no explosives or any arms had been found.
Bahri has been fighting extradition and is in custody until Dutch authorities take a decision on whether to send him to France.