Pakistani prime minister Shehbaz Sharif has declared a national day of mourning for citizens who died when the packed fishing trawler they were in sank off the coast of Greece.

Up to 750 men, women and children from Syria, Egypt, the Palestinian territories and Pakistan were on board the vessel, trying to reach relatives in Europe.

The Greek coastguard has defended its response to the tragedy that left more than 500 migrants presumed drowned. The vessel sank on Wednesday.

Survivors arrive by yacht at the port in Kalamata town
Survivors arrive by yacht into the port at Kalamata(www.argolikeseidhseis.gr via AP)

Mr Sharif expressed his grief over the tragedy and said Monday would be observed as a day of mourning, with the national flag flying at half-mast.

He previously said that Pakistan’s embassy in Athens had identified 12 nationals who were rescued by the coastguard. There was no official information on how many Pakistanis were onboard the vessel, how many survived or how many perished.

Greek authorities have been criticised for their failure to act more quickly. They said the migrants insisted they did not need any help, but non-governmental organisations said they received a number of calls for help.

Survivors of a shipwreck at a warehouse at the port in Kalamata town
Survivors at a warehouse in the port at Kalamata (www.argolikeseidhseis.gr via AP)

The sinking was one of the worst disasters of its kind this year.

Meanwhile, police in Pakistan-administered Kashmir said on Sunday that they had arrested 12 people involved in sending local youths to Libya for the onward journey to Europe.

Senior officer Khalid Chauhan said police had picked up the suspects in a crackdown on human traffickers.

Survivors of the shipwreck at a warehouse
Up to 750 men, women and children were onboard the vessel (Thanassis Stavrakis/AP)

Police were interrogating them for their alleged roles in luring, trapping and sending locals abroad after extracting huge amounts of money from them.

Around 28 people from the Koi Ratta area in the district of Kotli had gone to Libya for onward travel to Europe, police said.

Local official Chaudhry Haq Nawaz said there was still no confirmation on how many young men from the area were onboard the ill-fated boat, or how many were among the dead or missing.

A Greek coastguard vessel docked at the port in Kalamata town
A Greek coastguard vessel docked at the port in Kalamata town (Thanassis Stavrakis/AP)

He said efforts were under way to collect relatives’ DNA and the test results would be sent to Greece to help identify victims.

People have been offering their support to relatives of those presumed to have been on the boat.

Raja Sakundar, of Bindian village in Kotli, said his four nephews aged 18 to 36 remained missing.

“We were informed by the media (of the tragedy). When children are not found or die, you can understand what a parent goes through,” he said.

Raja Sakundar, right, holds a picture of his missing nephew on his phone
Raja Sakundar, right, holds a photo of one of his missing nephews (Nasir Mehmood/AP)

Raja Muhammad Majeed asked the Pakistani government to bring back his nephew, Raja Awais.

“If he is dead, bring back (the) body,” he said.

“When we bury him here, his mother, sisters and others can go to his grave and offer prayers. We will be patient.”