The manhunt for human traffickers has widened in Pakistan after a tragedy off the Greek coast last week left more than 500 migrants feared drowned, including many Pakistanis, officials said.

With no definitive casualty numbers announced, families feared for the fate of their loved ones and the nation observed a day of mourning on Monday, declared by Prime Minister Shahbaz Sharif’s government.

The Pakistani flag was flown at half-mast, and legislators in the national assembly, the lower house of the parliament, expressed their condolences.

A fishing trawler packed with migrants overturned and sank early last Wednesday off south-western Greece in one of the deadliest incidents of its kind in the central Mediterranean.

Grieving relatives
People offer their support to Raja Yousaf, right, whose son Raja Sajid is missing after a shipwreck off the Greek coast (AP)

The vessel was carrying as many as 750 people, including scores of Pakistanis, when it sank in international waters. A search and rescue operation is still under way.

Pakistan’s Embassy in Athens has so far identified 12 Pakistani nationals rescued by the coast guard.

Interior minister Rana Sanaullah Khan said the country’s Federal Investigation Agency has launched a crackdown against human smugglers, arresting key suspects in the eastern city of Lahore and Karachi, the capital of southern Sindh province.

“All the people involved in this tragedy will be brought to justice,” the minister pledged in a statement, adding that Mr Sharif’s government will further toughen existing laws to include harsh punishments for human traffickers.

So far, authorities have detained nearly two dozen suspects, including two key suspected traffickers in Pakistan, and at least 12 people involved in sending young men to Libya for the onward journey to Europe.

Grieving relatives
Raja Sakundar, right, holds up a picture of his missing nephew on his phone (AP)

Senior Pakistani officer Khalid Chauhan said police had picked up the suspects amid the crackdown on traffickers and were interrogating them for their alleged roles in luring, trapping and extracting huge amounts of money to send the men abroad.

Abdul Jabbar, a top official at the Federal Investigation Agency, appealed to families of those who died in the boat incident or went missing to come forward and share information about the smugglers.

He promised the families of the victims would not be arrested but “rather they will be treated as victims” themselves.

Pakistani officials also collected DNA samples from relatives and said they would send the results to Greece to help identify victims.

Since the news of the incident spread, people have been offering their support to relatives of those presumed to have been on the boat.

Some of the survivors have contacted their families, telling them how the boat sank.