The migrant rescue ship Aquarius will not be allowed to sail under the Swiss flag until there is a common European migration policy, the government in Bern said Monday as it rejected a parliamentary proposal.

The Aquarius, run by the charities SOS Mediterranee and Doctors Without Borders (MSF) has been blocked in Marseille since early October.

It was removed from Panama's shipping register the same month, leaving it flagless and unable to sail at sea.

Four green, social democratic, conservative and liberal Swiss legislators had asked their government to grant the ship the Swiss flag, appealing to Switzerland's humanitarian tradition.

They said that the vessel has saved some 29,000 lives and is the last remaining non-governmental migrant rescue ship in the Mediterranean.

A parallel Swiss online petition has been signed by some 35,800 people.

The government countered Monday that "rescue operations in the Mediterranean require a coordinated and long-term approach" among European countries.

"Such a solution is not yet a reality," Bern said in its response, pointing to the reluctance of European coastal countries to let migrants ashore.

"In this context, isolated actions risk compromising rather than encouraging the necessary cooperation between states," the government concluded.

SOS Mediterranee and MSF have accused Italy of putting pressure on flag states to deny registration to the Aquarius.

Italy's Interior Minister Matteo Salvini, who has banished migrant rescue boats from Italian ports, has denied the claim.

However, Rome has sought to impound the Aquarius on charges of illegal waste disposal.