Spain's prime minister says words aren't enough anymore and he urged the European Union to take swift action after a boat believed to be crowded with about 700 migrants capsized north of Libya overnight.

There are 24 people confirmed dead, but there are fears that hundreds of others are missing, in what could become the Mediterranean's deadliest known migrant sea disaster.

"Today, and this is the umpteenth time, we hear of yet another human tragedy in the Mediterranean, off the Libyan coast," Spanish Prime Minister Mariano Rajoy told a political rally Sunday. "It's a daily drama. Three days ago it was 400 people. Four days ago they were 19.

"It's a response that has to come from Europe," Rajoy said. "Words won't do any more. We have to act, and as Europeans we are gambling with our credibility if we aren't able to stop these dramatic situations that are now happening on a daily basis."