The UK police force looking into the deaths of 39 people found in a refrigerated truck on Wednesday have appealed to people who may be in the country illegally to help their investigation. 

In a news briefing on Thursday, deputy chief constable Pippa Mills said that they could come forward without any threat of legal action - or could telephone a confidential police hotline anonymously.

She also urged families who suspected they may know the victims to get in touch. 

Meanwhile a fourth person - a 48-year-old man from Northern Ireland - has been arrested at Stansted Airport in connection with the investigation.

Earlier on Friday Essex Police said a man and woman, both in their thirties, had been arrested in Warrington in Cheshire on suspicion of manslaughter and conspiracy to traffic people. The driver of the truck remains in custody on suspicion of murder.

^{Tong Xuejun, Minister Counsellor at the Chinese Embassy, said international action was needed to prevent such a tragedy happening again} 

The UK police on Thursday said that they believed the victims were Chinese nationals, but there have been mounting reports that at least some of the victims are thought to be Vietnamese.

Deputy chief constable Mills said that they had given "an initial steer" on nationality "but it was a developing picture."

She urged people not to speculate on the identities or nationalities of the victims, and said that the force would not be making any comment on the various reports. 

The Chinese Foreign Ministry earlier said it hoped the United Kingdom could confirm the nationalities of the victims in the incident and punish the criminals involved in the case.

Foreign Ministry spokesperson Hua Chunying expressed sorrow over the deaths, saying the Chinese government was paying a lot of attention to this case and was in close contact with local authorities. 

The global community should pay attention to the illegal immigrants issue and strengthen cooperation to prevent such tragedies from happening again, said Hua at a press conference. 

Eight women and 31 men were found dead inside a refrigerated container lorry in Grays, UK on Wednesday.

The investigation is still under way. British authorities are conducting autopsies on the victims as they begin the process of identifying them and establishing how and when they died.

China's ambassador to the UK Liu Xiaoming tweeted: "Our thoughts are with the 39 victims in #Essex no matter where they are from. This has again drawn world attention to the issue of illegal immigration. The world should join hands and take resolute measures to prevent such tragedy from happening again."

The Vietnamese embassy in London said families from the country had got in touch about missing relatives. There were growing concerns at least 10 of the victims could have been from Vietnam, reported Reuters.

The tractor part of the truck  entered the UK via Holyhead in North Wales on October 20, having traveled there from Dublin.

However, the container in which the bodies were found had traveled from Zeebrugge in Belgium to the port of Purfleet in Essex, England, at around 00:30 local time on October 23. The cab picked up the trailer and left the port shortly after 01:05 local time on Wednesday.

Emergency services arrived at Waterglade Industrial Park soon after, having been alerted to the incident.