Twenty migrants were missing in the Mediterranean after their vessel sank off the Italian island of Lampedusa, a senior official with the UN refugee agency said on Wednesday.

"Twenty people are reported missing in the Mediterranean after a shipwreck on September 1," Chiara Cardoletti, the UNHCR official for Italy, posted on X.

"The seven survivors, taken in by our team on Lampedusa, are in a critical condition," she said, adding that several of them had lost loved ones in the disaster.

Some Italian media reports put the number of missing at 21, including three children.

The survivors, Syrians, were rescued by the Italian coastguard.

In 2023, more than 3,000 migrants were reported missing after having attempted the perilous Mediterranean crossing from north Africa, according to the International Organisation for Migration (IOM). (AFP)