More than 700 people are feared dead after a fishing boat crammed with migrants seeking a better life in Europe capsized off Libya, with some survivors suggesting nearly 1,000 could have been on board.
"With this latest tragedy... We have no more excuses, the EU has no more excuses, the member states have no more excuses," Mogherini said.
"We need immediate action from the EU and the member states," she said as she arrived for an emergency meeting of foreign and interior ministers to discuss what could be done.
The Bruno Gregoracci coast guard cutter docked with 24 body bags laid out on its deck, which were then carried to waiting hearses for transfer to a morgue.
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The deaths of hundreds more desperate migrants caused an outcry across Europe, where newspapers called it the "EU's darkest day" and a "disgrace" for EU policy.
"European leaders knew full well that the number of victims at sea from African migration would smash records from the spring," Belgium daily Le Soir wrote.
"The world needs to react with the conviction with which it eliminated piracy off the coast of Somalia a few years ago," said Wiliam Lacy Swing, director general of the International Organization for Migration.
"All of us, especially the EU and world's powers can no longer sit on the sidelines watching while tragedy unfolds in slow motion."
Malta's Prime Minister Joseph Muscat said the EU had to address the chaos in Libya which allows smugglers to operate out of the troubled north African state with impunity.
The foreign ministers on their own had been due to discuss the situation in Libya.
Mogherini, a former Italian foreign minister and very aware of her country's anguish over events, has been pushing the EU to be more pro-active on Libya and she made the point again.