The UN's refugee agency chief hailed a Franco-British deal agreed Thursday to tackle people smuggling, but said increasing "legal avenues" to enter Europe was key to any solution on human trafficking.

In a statement, the UN's High Commissioner for Refugees Antonio Guterres praised elements of the deal that boost assistance for migrants and refugees, including thousands living in slum-like conditions in the northern French port city of Calais.

Under terms of the deal announced earlier on Thursday, Britain and France said they would create a new command and control centre to bust smuggling rings.

While recognising "the importance of combatting smuggling and trafficking", Guterres said it was necessary to increase the legal ways for migrants and refugees to enter the continent.

"It is important to note that to be effective in cracking down on smugglers and traffickers, we must increase the number of legal avenues for people in need of protection to come to Europe," he said.

His comments echoed earlier remarks by Red Cross chief Elhadj As Sy who told AFP the only way to stop traffickers was to increase legal means of migration.

Refugees have this summer made numerous attempts reach Britain via the Channel Tunnel, some of whom have died on route.

The UN has fiercely criticised the response of European governments to the growing migrant crisis, saying that far more resources must be committed to dealing with the influx of new arrivals.

Britain's handling of asylum requests from people with legitimate claims to refugee status have specifically been criticised by senior UN officials.

Speaking before the Franco-British agreement was signed, the Red Cross chief said he hoped the Calais deal would not go "overboard into punitive measures", targeting the vulnerable people camped out in city.