The presumed drowning of 400 migrants in the Mediterranean Sea is one of the deadliest known migrant tragedies in the last decade, the UN refugee agency has said, as the Italian coastguard reported it has rescued nearly 10,000 migrants in less than a week.

Amid an unprecedented wave of migration towards Europe from Africa and the Middle East, the Italian coastguard rescued about 140 people from the shipwreck off the coast of Libya on Monday, and recovered nine bodies.

Survivors told the aid group Save the Children, however, that an estimated 400 people are believed to have drowned.

"According to their stories, they all departed from Libya, more than 550 people on the same boat that capsized only 24 hours after they departed," Carlotta Bellini, a Save the Children spokeswoman in Rome, told Al Jazeera.

The coastguard said it assumed that there were many dead given the size of the ship.

The rescue was made during a five-day surge that saw Italian ships rescue nearly 10,000 people at sea since Friday - an unprecedented rate in such a short period, according to Filippo Marini, a coastguard spokesman, the AP news agency reported.

The UNHCR estimates 3,500 migrants died in the Mediterranean last year, up from 600 in 2013. With few bodies recovered, many deaths are never officially confirmed. Instead, their fates are recounted by survivors and, in cases when boats are lost at sea without any rescue attempt, by relatives who report their failure to arrive in Europe.

To date this year, the number of dead or missing at sea is 900, according to the UNHCR, compared with just 17 the same time last year. Typically, the arrivals of migrants making the perilous journey aboard smugglers' boats goes up in April as the weather improves, increasing concerns about the coming months.

EU criticised by rights groups

Rights groups criticised the EU for scrapping rescue operations in the Mediterranean, saying it had endangered the lives of thousands of desperate migrants making perilous journeys across the sea.

"European governments' ongoing negligence towards the humanitarian crisis in the Mediterranean has contributed to a more than 50-fold increase in migrant and refugee deaths since the beginning of 2015," Amnesty International said in a statement.

William Spindler, a specialist on asylum seeker and refugee issues at the UN's refugee agency, said that due to conflict in places like Syria and the Horn of Africa, the number of people trying to find safety in Europe has increased "enormously" since last year.

Spindler said that to end the tragedies at sea, people smuggling needs to be combated, and the capacity to rescue people at sea increased.

"At the same time we need to open the possibility for refugees to come legally to Europe so that they don't need to take these dangerous journeys," he told Al Jazeera.

"And very importantly, we need to help countries that are hosting the vast majority of refugees in the world, countries such as Lebanon, Jordan, Sudan, Ethiopia, and Kenya... We need to make sure they can continue to keep refugees safe - because otherwise refugees will continue these journeys and risk their lives to find safety in Europe."

Source: Al Jazeera and agencies