Over 430 people rescued in the Mediterranean prepared to disembark in Sicily on Thursday as two charity ships were authorised to dock just hours after one issued an ultimatum to Italy.

The Mare Jonio rescue ship, owned by the Italian NGO Mediterranea, told the interior ministry Wednesday it would "enter the first accessible port" without permission if its requests for a safe harbour were not answered within 10 hours.

The vessel was carrying 92 people rescued in two operations on June 5 and 6 in international waters around Libya and Malta, including some 30 unaccompanied minors.

It was assigned the port of Pozzallo in southern Sicily and was set to disembark those rescued on Thursday.

Fellow German charity Sea Watch, which on Wednesday had said its vessel Sea Watch 3 was also still waiting for a port after four days, with 352 people on board, was also assigned Pozzallo.

"After exhausting days, finally a port of safety for the #SeaWatch3", it said Thursday on Twitter.

Nearly 120 of those rescued by the Sea Watch were unaccompanied minors.

Charities have accused Italy's government of deliberately being slow to assign ports to their rescue vessels in a bid to dissuade people from attempting the dangerous crossing from north Africa.

SOS Mediterranee, which has rescued nearly 1,000 migrants since the beginning of the year, called Wednesday for the urgent adoption of a European solidarity mechanism to avoid such delays.

Its Ocean Viking vessel rescued some 300 people, including a three-month-old baby and six pregnant women, on its last mission in May, but had to wait up to 10 days to enter a safe port.

Some 17,000 people have crossed the central Mediterranean -- considered the world's most dangerous migrant crossing -- since the start of the year, according to Italy's interior ministry.