Pope Francis and UN Secretary General Ban Ki-moon on Tuesday voiced shock over migrant deaths in the Mediterranean and agreed on the need for tackling global warming, just weeks before the Vatican issues an official opinion on the controversy.

Talks on the sidelines of a Vatican seminar on environmental issues were largely taken up by the migrant crisis, an issue on which both leaders have campaigned for a change of approach from the international community.

"We shared our shock at the recent grievous loss of life in the Mediterranean," Ban told a press briefing, calling on European Union governments to step up measures to prevent avoidable deaths while working to address the root causes of the accelerating flow of asylum seekers and economic migrants trying to reach Europe by sea.

Nearly 5,000 migrants have drowned in waters between Libya and Italy since the start of 2014 with the death rate accelerating significantly since a major Italian search-and-rescue operation was suspended late last year because of cost pressures and opposition from other EU states who said it only encouraged migrants to attempt the journey.

"These migrants, many of whom are refugees, are desperate for a better future," Ban said. "We need to strengthen search and rescue operations, and stop the criminals who exploit the most vulnerable people. We need to address the roots of the problem."

EU leaders agreed last week to triple funding for the bloc's maritime border patrol, but aid organisations say the resources devoted to life-saving missions remain inadequate with no end in sight to the surge in the numbers of migrants trying to reach Europe from conflict zones such as Syria, repressive states like Eritrea and desperately poor regions like much of sub-Saharan Africa.

"The countermeasures (agreed by EU leaders last week) are an important first step... but more needs to be done," Ban said.

"Thousands of people are drowning. These are the poorest and most vulnerable people, who are risking their life... for the slimmest possibility of an opportunity. That really humbles and saddens us."

Ban dismissed an idea, supported by the Italian government, of destroying boats that could be used by traffickers operating out of Libya, suggesting it could be disastrous for the local fishing-based economy.

"The priority should be given to protecting human rights and dignity and to saving human lives," Ban said.

"Targeting and destroying these boats is not a good way."

- Environmental accord? -

The two leaders also touched on environmental issues, just weeks before Francis publishes a de facto policy document on environmental issues that could have a profound impact on the debate on global warming.

Addressing the Vatican seminar, Ban said his talks with the pontiff had been "fruitful and wide-ranging" and that he was looking forward to the upcoming encyclical from the Church, expected in June or July.

An encyclical is a statement of fundamental principles designed to guide Catholic teaching on a subject. It is issued in the form of a letter from the pope to bishops around the world.

"Science and religion are not at odds on climate change," Ban said. "Indeed they are fully aligned."

Campaigners on climate change believe that an expression of concern from the Church about the impact of global warming, and a signal of support for steps to reduce the fossil fuel consumption they believe causes it could have great influence on the global reflection on the scale of the problem, its roots and what, if anything, needs to be done.

Pope Francis is due to address the UN Special Summit on Sustainable Development in September and the international community will seek to agree a universal agreement on climate change at a summit in Paris in December.

"Paris is not the end point, but must be a turning point in finding a common way forward in meeting the climate challenge," Ban said.

Climate change sceptics have warned Francis that it would be folly for the Catholic church to integrate the UN's view of global warming into its official teaching, arguing that the views of people like Ban are not grounded in reliable science.