The UN's new human rights chief has urged Egypt to join with European nations in hunting down the human traffickers who allegedly drowned up to 500 migrants in the Mediterranean by ramming their boat. He said the traffickers are likely guilty of "mass murder" and should be brought to justice.

Zeid Ra'ad Zeid Al Hussein, the UN high commissioner for human rights, said it was important to "end the prevailing impunity surrounding such crimes", stressing that those seeking escape from chaos and warfare, such as the huge numbers fleeing Syria, had the right to seek refuge. He also demanded a wider rethink of the conditions that push so many thousands to undertake such risky voyages.

The first accounts of the events on a migrant sailing between Egypt and Malta emerged on Monday, when two Palestinian men, among a handful of survivors, told investigators from the International Organisation for Migration (IOM) that the people smugglers deliberately sank a boat carrying up to 500 people after the migrants refused to move to a smaller vessel. The story was later corroborated by other survivors, who said between 50 and 100 children might have died when the traffickers used their boat to ram the migrants' craft.

Describing it as "a truly horrendous incident", Zeid said in a statement: "It is the duty of states to investigate such atrocious crimes, bring the perpetrators to justice, and even more importantly to do more to prevent them from happening in the first place. All the countries in the Mediterranean must make a concerted effort to clamp down on the smugglers who are exploiting one of the most vulnerable groups on the planet and endangering their lives, virtually on a daily basis, purely for financial gain.

"The callous act of deliberately ramming a boat full of hundreds of defenceless people is a crime that must not go unpunished. If the survivors' accounts are indeed true – and they appear all too credible – we are looking at what amounts to mass murder in the Mediterranean."

If such a mass murder was committed on its own soil any country would "throw the full weight of their police forces and justice systems behind an investigation", said Zeid, a Jordanian diplomat who took over the UN role at the start of this month. "The reaction should not be any less rigorous just because the victims are foreigners and the crime took place on the high seas. Yet very few people who kill, rape or rob migrants during their journeys end up in court."

According to survivors, the migrants, comprising a number of Palestinians as well as nationals from Egypt, Syria, Sudan and Eritrea, boarded a boat in the Egyptian port of Damietta and switched ships three times during a four-day voyage. When they refused to join the last boat, fearing it was unseaworthy, the traffickers became enraged and sank their vessel.

Zeid said the smugglers are all believed to be Egyptian or Palestinian. He urged officials in Malta, as well as Greece and Italy where the survivors were taken, to share information with Egyptian authorities. He said: "You cannot transport large quantities of foreigners in buses into a major port and cram them on board a ship without the port authorities and other witnesses being aware of what is going on."

Urging a wider rethink of migration issues, he said: "Far too many refugees and migrants are dying all across the world in an effort to flee conflict, systematic political oppression and human rights violations, including economic deprivation. These root causes in their countries of origin must be tackled in a concerted manner."