The UN Children's Fund (UNICEF) on Sunday expressed alarm at the number of migrant and refugee deaths in the past week in the Central Mediterranean, many of whom were believed to be unaccompanied minors.

In anticipation of a major summer upswing of child migrants using the dangerous crossing between Libya and Italy, UNICEF will shortly begin an operation with the Italian government and partners to provide protection support, the UN agency said in a press release.

The UN agency made the expression after at least 700 migrants may have died while trying to cross the Mediterranean to reach Europe, the UN's refugee agency (UNHCR) said on Sunday. Hundreds of others are missing after their boats capsized in the waters.

The vast majority of children using the crossing are unaccompanied adolescents and they have faced appalling abuses, exploitation and the possibility of death at every step of their journey.

"The stories which I have personally heard from children making this journey are horrifying. No child should face them. Their lives are in the hands of smugglers who care for nothing other than the money they exhort from them," said Marie-Pierre Poirier, UNICEF special coordinator for the European Refugee and Migrant Crisis.

An average of 1,000 unaccompanied children a month have arrived in Italy this year but UNICEF expects this figures to spike in the coming months.

UNHCR said on Sunday that several shipwrecks had taken place over the weekend as migrants tried to cross the Mediterranean Sea in flimsy vessels.

Nearly 600 people were missing after boats capsized on Wednesday and Thursday, the refuge agency said.