More than 30 people have drowned after about 200 people fell from an overcrowded migrant boat off the coast of Libya, the latest tragedy in the Mediterranean Sea.

The vessel was carrying between 500 and 700 people when the accident happened 20 nautical miles off the Libyan coast. It was not clear whether those in the water were knocked off balance by a wave or because there was a rush to one side of the boat as a rescue vessel approached.

The crew of the Phoenix aid boat, chartered by the Maltese NGO Moas, began a rescue operation and were distributing lifejackets. The number of deaths remains unclear.

The Italian coastguard, which was coordinating rescues, dispatched other vessels to the area to look for survivors.

“It’s not a scene from a horror movie, it’s a real-life tragedy that is taking place today at the gates of Europe,” Chris Catrambone, who works with Moas, told AFP. Catrambone, who gave updates on the rescue operation on his Twitter feed, said 31 bodies had been recovered.

not a scene from a horror movie...Real life tragedy unfolding on Europe's doorstep today! pic.twitter.com/OcJH8C92Yh — Chris Catrambone (@cpcatrambone) May 24, 2017

The incident was one of 15 rescue operations over the past 24 hours, which have saved up to 1,700 people. There is usually a spike in the spring and summer months in people attempting the perilous crossing to Europe, because of warm weather and calm seas. In the year up to 17 May, 45,754 people departing from the Libyan coast had been rescued at sea and taken to Italy, and more than 8,500 of them had arrived in the last two weeks. The figures represent a big increase on the same period last year.

The EU is training the Libyan coastguard to step up its efforts to turn back boats if they are inside Libyan waters, but aid groups have accused Libyan authorities of subjecting migrants to violence and other abuses if they are returned.

On Tuesday, the German NGO Jugend Rettet, which has been working in rescue operations in the central Mediterranean, claimed that the crew of a speedboat labelled “Libyan coastguard” fired shots at migrant boats during a rescue operation on Tuesday.

In a statement on Facebook, Jugend Rettet said its staff witnessed multiple gunshots being fired from the speedboat. “[They] were seemingly aimed at the people on the boats ... Furthermore several boats were pushed back into Libyan waters ... This is a violation of the non-refoulement principle – thereby of human rights,” the statement said.

The captain of the NGO’s vessel added: “About 100 people panicked and jumped from the boats ... We cannot say whether and how many dead there were in the shooting. We had to be careful not to get a bullet ourselves. We are speechless against this crude violence.”

+++Breaking+++ On this photo on the right side you can see the guns by Libyan Coast Guards pointed on the refugees! pic.twitter.com/P4vuj4oQzQ — Jugend Rettet e.V. (@jugendrettet) May 23, 2017

+++Breaking+++Around 100 people jumped into the water in panic to reach @SOSMedGermany and us. The situation was under control before shoots — Jugend Rettet e.V. (@jugendrettet) May 23, 2017

+++Breaking+++We repeat: SAR crews are save. We had to step back because of the shoots and are waiting for more information from our crew! — Jugend Rettet e.V. (@jugendrettet) May 23, 2017

+++Breaking+++ Several NGOs are witnessing huge human rights violations. People's lives are put into direct danger by boat marked as LCG! — Jugend Rettet e.V. (@jugendrettet) May 23, 2017

One journalist reporting on migrant boast rescues said that rescue vessels and migrant boats faced “severe harassment” from the Libyan coastguard, which fired warning shots near them.

At one point, perhaps a dozen or more migrants jumped into the water in panic, endangering their lives. — Richard Hall (@_RichardHall) May 24, 2017

In October, the humanitarian group Sea-Watch, based in Germany, said the crew of a speedboat labelled “Libyan coastguard” attacked a migrant boat carrying about 150 people, beating them with sticks and causing many to fall into the water and at least four to drown. Last week Sea-Watch called on the international criminal court to extend the ongoing investigation of human rights violations in Libya to the activities of the Libyan coastguard.

The EU and Italy agreed in February to fund migrant centres managed by UN agencies, but progress has been slow due to the security crisis. On Monday, the UN refugee agency also called on the Libyan government in Tripoli to close its refugee detention centres, describing conditions as inhumane and shocking.

The UN high commissioner for refugees, Filippo Grandi, said after a visit to Tripoli that he was “shocked at the harsh conditions in which refugees and migrants are held” in Libya, which he said was “generally due to lack of resources”.Amnesty International has said it is “deeply concerned that Italian authorities may be attempting to circumvent their obligation to protect people fleeing widespread and systematic human rights violations and abuses in Libya by facilitating the interception of refugees and migrant boats by Libyan authorities in the central Mediterranean.”