ROME — More than 250 people, including women and children, were missing on Wednesday after their boat capsized off the coast of the Italian island of Lampedusa in the worst shipwreck since thousands of migrants began traveling to Italy because of the unrest in North Africa.

The vessel, which capsized in the predawn dark, had been carrying mostly sub-Saharan Africans, including at least one pregnant woman. The capsized vessel was among the first to try to reach Lampedusa from Libya since fighting began in Libya in February. About 22,000 migrants have arrived on the island since January, most of them from Tunisia.

The shipwreck is an alarming sign of the challenges facing Italy as it contends with a wave of migrants leaving North Africa. Aid officials on Wednesday called for a swifter response in boat rescues and better coordination among Mediterranean countries in distinguishing between “economic immigrants” seeking work in Europe and refugees seeking asylum.

After interviewing survivors on Lampedusa, Italian officials and the International Organization for Migration estimated that more than 250 people were missing.