The horrific deaths of up to 500 migrants in a deliberate act of ramming and sinking their boat in the Mediterranean could amount to mass murder and the perpetrators must be tracked down and punished, according to the UN human rights top official.

Zeid Raad al-Hussein urged European and African states to find the smugglers responsible for the incident and treat them as members of criminal gangs who kill citizens within their borders.

"The callous act of deliberately ramming a boat full of hundreds of defenceless people is a crime that must not go unpunished," he said in a statement.

"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."

He also called for nations to tackle the root causes of the migration problem.

According to survivors' reports, the human traffickers were apparently laughing as they deliberately sunk the boat packed with asylum seekers. Two Palestinian migrants, who were rescued by a freighter after spending more than two days in the water, confirmed to the International Organisation for Migration (IOM) initial reports that up to 500 people died in the incident.

They told IOM investigators that the 10 traffickers, believed to be Palestinian and Egyptian, turned against the migrants as they refused to switch to a smaller, unseaworthy boat on the way to Europe.

At first, smugglers threatened to return everyone to the port of origin in Egypt.

When this failed to convince the would-be-refugees, who had already switched boat three times since they had left the port of Damietta few days earlier, the traffickers started yelling and throwing sticks at them, the survivors said.

Finally they rammed their vessel into the other boat causing hundreds to drown.

The incident happened at the end of last week, about 300 miles southeast of Malta.