Pope Francis has condemned the conditions of workers who died in a Bangladesh factory collapse last week as "slave labour," saying unjust salaries and the unbridled quest for profits were "against God".

The pope's words were his toughest yet on workers' rights since his election on March 13, another indication that the former archbishop of Buenos Aires is intent on making social justice a major plank of his pontificate.

"Living on 38 euros ($50) a month - that was the pay of these people who died. That is called slave labour," Francis said in a private impromptu sermon at his personal morning Mass in his residence, Vatican Radio reported on Wednesday.

The death toll from the collapse of the illegally built Rana Plaza in Dhaka's commercial suburb of Savar rose to 430 on Thursday.

Speaking on May Day, the international labour day, the pope said: "Not paying a just wage, not giving work, only because one is looking at the bottom line, at the budget of the company, seeking only profit - that is against God".

Francis, the former Cardinal Jorge Bergoglio of Argentina, said there were many people in the world living in conditions of slave labour.

"Today in the world there is this slavery that is perpetrated with the most beautiful thing that God has given man: the capacity to create, to work, to make his own dignity," he said.

"How many brothers and sisters in the world are in this situation because of these economic, social and political policies?"

Cheap fashion

In his native Argentina, Francis was often on the side of the poor, the downtrodden and the unemployed, clashing with the government on economic policy and defending the dignity of the weakest members of society.

Duty-free access offered by Western countries and low wages have helped turn Bangladesh's garment exports into a $19 bn a year industry, with 60 percent of clothes going to Europe, where the tragedy has raised questions about the human cost of cheap fashion.

The International Labour Organisation estimates that almost 21 million people worldwide are victims of slavery or forced labour.

Almost half are thought to be trafficked, either across borders or within their own countries.