(CNN) Calm seas and good weather in the southern Mediterranean are prompting a wave of migrant smuggling ships to attempt the crossing from Libya to Italy, the International Organization for Migration said Sunday, putting many lives at risk.

European navies and nongovernmental organizations are struggling to locate and rescue migrants from the rickety ships, and thousands may be pulled to safety Sunday, a spokesman for the organization told CNN. "The numbers are high, and they are rising," Federico Soda said.

The warning comes after nearly 3,500 migrants were rescued Saturday.

The HMS Bulwark, a British Royal Navy ship, has rescued more than 1,000 migrants on Sunday alone, including 10 pregnant women, a spokeswoman for the UK Defence Ministry told CNN.

It is the largest rescue operation the Bulwark has had since deploying to the Mediterranean on May 5, and the numbers are climbing as the operation is still ongoing, she said.

The ship has rescued more than 2,700 migrants since it was sent to the Mediterranean.

A spokesman for the U.N. refugee agency, UNHCR, said naval ships from Italy and Spain were also involved in the effort to rescue migrants on Sunday, along with the Italian coast guard.

The Italian coast guard has received requests for help from 14 vessels in distress, carrying an estimated 1,500 refugees and migrants, the UNHCR's William Spindler said.

They have rescued migrants from 11 vessels, and operations to find the other three boats and rescue those on board continue.

Commercial vessels are also being directed to the rescue area to help with the operation, Spindler said.

A spokesman for the Italian coast guard, Andrea Sbalbi, earlier said 10 rescue operations were ongoing for migrants at sea in wooden fishing boats and rubber dinghies.

Migrants taken to Sicily

Two German naval ships that coordinated Saturday's rescue of thousands of migrants north of the Libyan coast are on their way to Sicily to drop them off, a spokesman for Germany's Bundeswehr Joint Forces Operation Command told CNN on Sunday.

The two ships, the Berlin and the Hessen, picked up 1,411 migrants from four vessels Saturday -- 939 men, 327 women and 145 children. In just one vessel there were 563 migrants, the spokesman said.

The Hessen was expected to dock at Palermo and the Berlin at Trapani on Sunday. The ships will be replaced early next week by two new ships from the German navy that are on their way to the Mediterranean.

The Irish navy ship LE Eithne, which was involved in Saturday's rescue operation, is returning to Italy with 399 migrants on board -- 280 men, 78 women and 41 children, a spokeswoman for the Irish Defence Forces told CNN on Sunday. The ship is not involved in Sunday's ongoing rescue operation in the Mediterranean.

According to the Italian coast guard, the ship is due to bring the migrants to Taranto, in Italy's Puglia region, early Monday.

Joint effort

A dozen ships from several countries, along with at least one private ship, were involved in the rescue operations Saturday, the Italian coast guard said.

Photos: Amanpour joins migrant rescue operation in the Mediterranean Photos: Amanpour joins migrant rescue operation in the Mediterranean CNN Chief International Correspondent Christiane Amanpour was onboard an Italian Navy vessel Wednesday as it rescued hundreds of desperate African refugees and migrants trying to reach Europe via the Mediterranean Sea Hide Caption 1 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean A sigh of relief: Women and children sleep on the deck of the Sfinge, an Italian Minerva-class warship -- "sfinge" is Italian for "sphinx" -- after being rescued.



Producer Dominique van Heerden took these photos after the CNN team helicoptered in from the Italian island of Lampedusa. Hide Caption 2 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean Everyone who participated rescue had to suit up in full protective gear in case any of the refugees and migrants were carrying diseases -- that rule was no different for Amanpour. Hide Caption 3 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean



Tens of thousands of migrants have attempted the perilous journey across the Mediterranean this year, and at least 1,826 have died, Amanpour speaks with migrants, almost all from Eritrea, onboard the Sfinge.Tens of thousands of migrants have attempted the perilous journey across the Mediterranean this year, and at least 1,826 have died, according to the International Organization for Migration , many times more than had perished during the same period last year. Hide Caption 4 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean There were some 290 people onboard the rescued boat, including 35 children, Amanpour reported.



One man was dead by the time the Navy got to the boat, and three were so ill that they were helicoptered to Lampedusa for urgent care. Hide Caption 5 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean A woman shields her five-year-old child from the intense Mediterranean sun onboard the Sfinge. Hide Caption 6 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean The Italian Navy spotted the medium-sized wooden boat that carried the refugees and migrants some 85 miles from Lampedusa. Hide Caption 7 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean The rescuers know that beautiful weather on the Mediterranean means it will be a busy day, as hundreds of people set off from the North African coast -- mainly Libya -- bound for European shores. Hide Caption 8 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean Surrounded by water with nothing to drink: Dehydration can be a serious problem for those who attempt the sea crossing. Here, a man drinks after being brought onboard by the Italian Navy. Hide Caption 9 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean Rescuing the migrants is an "extraordinarily complex process," Amanpour reported.



"It takes quite a long time to rescue boat of 300 people. They have to check, they have to make sure that there's nothing hostile on board."



"Then all the naval personnel suit up in white hazardous material suits with masks and gloves -- we all had to do the same thing -- in order to make sure that you don't get contaminated by any potential disease." Hide Caption 10 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean The only way to communicate with the outside world from the middle of the sea is satellite phone. Here, Amanpour stands on the bridge of the frigate Virginio Fasan while she speaks with CNN's Hala Gorani Hide Caption 11 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean Amanpour speaks with Rear Admiral Pierpaolo Ribuffo, Deputy Commander of Italian Navy and commander in charge of the Taskforce Operation Mare Sicuro, onboard the Italian frigate Virginio Fasan. Hide Caption 12 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean Amanpour speaks with Marco Bagni, commanding officer of the Italian frigate Virginio Fasan. Hide Caption 13 of 14 Photos: Amanpour joins migrant rescue operation in the Mediterranean The sun sets over the Sfinge rescue ship. Hide Caption 14 of 14

Ian Ruggier, a member of the humanitarian group Migrant Offshore Aid Station , was on one of the rescue boats.

Some rescue ships turned up with rescued migrants already on board, according to Ruggier. That's what happened with one German naval vessel that picked up 301 migrants from an unreported vessel as the German ship was on the way to the larger rescue scene, a spokesman for Bundeswehr Joint Forces Operations Command said.

The German ships went to the main site and found seven packed boats -- far more than they expected -- according to the spokesman for the command, which is part of Germany's military.

The Italian Interior Ministry reported that several of its ships were involved, including from its coast guard and navy. Some Italian rescue vessels reached the island of Lampedusa on Saturday night.

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Europe dealing with immigration crisis

The prospect of migrants being in jeopardy in the Mediterranean is serious but hardly surprising. People from impoverished and conflict-torn countries such as Syria, Somalia, Sudan and Eritrea have taken to the sea to reach Europe in especially large numbers of late, often putting their fate in the hands of people-smugglers.

The United Nations estimates that, as of the end of May, 90,000 refugees and migrants had crossed the Mediterranean into Europe so far this year. Just over half landed in Italy, with roughly 42,000 in Greece and the rest recorded in places such as Spain and Malta. The UNHCR's Spindler said that about 1,850 had died or were missing at sea.