Up to 700 migrants are believed to have died in the past week crossing the Mediterranean Sea, the United Nations said Tuesday.

At least three boats sank in the biggest death toll for migrants in more than a year, the United Nations Refugee Agency (UNHCR) said.

The boats left Libya and were trying to reach Italy. Most of the migrants came from Africa.

About 14,000 people were rescued in calmerseas since Monday, Reuters news agency reported. Survivors who arrived in Italy said one of the boats was carrying many women and children.

The migrants pay smugglers to take them on small rubber boats or old fishing vessels. Many do not know how to swim and do not have life jackets.

Giovanna Benedetto is with the humanitarian group, Save the Children. She said the survivors spoke about horrible conditions on the boats.

"They lived [in] terrible situations. They told us terrible stories. They saw killing people – only because those people want to have some food. Their stories are really, really terrifying."

The U.N. announced Tuesday that more than 2,500 migrants have died trying to cross the Mediterranean Sea into Europe so far this year.

That is a big increase from 1,855 people who died in the first five months of 2015.

The trip from North Africa to Italy is the deadliest way to cross, according to UNHCR spokesman William Spindler.

"2,119 of the deaths reported so far this year have been among people making this journey, making the odds of dying as high as one in 23," he said.

Most migrants traveling this way are from Nigeria and Gambia. Spindler said the migrants are risking everything to make the trips because they face very desperate conditions at home.

"The reason why so many people are taking to sea at the moment, and risking their lives, is because they have no option. We have to give them the possibility to travel legally and safely."

I'm Bryan Lynn.