Pope Francis has slammed "state terrorism" that "kills the innocent at the same time as terrorists", just hours after he took a swipe at the "haggard" state of Europe.

It was not clear whom the pontiff was referring to in the remarks made on his plane returning to the Vatican from a visit on Tuesday to the European Parliament and Council of Europe in Strasbourg, France, but there was little doubt they would make waves.

The 77-year-old Argentinian pontiff was responding to a complex question about the threat posed by groups like Islamic State in Syria and Iraq and the scourge of modern slavery.

"There is another threat, that of state terrorism" the pope said, bemoaning what happens when international crises degenerate.

"Each state, for its own part, feels it has the right to massacre terrorists. But so many innocent people perish at the same time as the terrorists," he said.

"It is a type of high-level anarchy which is very dangerous."

The pope's comments could - and almost certainly will be - interpreted as referring to action by a number of governments, including Syria's attempts to crush Islamist rebels, US action against Islamic State (IS) and its drone strikes in Afghanistan/Pakistan, or Israeli operations against Palestinian militants.

Vatican experts suggested they could be an attempt to rebalance remarks the leader of the Catholic church made in August on his return from a visit to South Korea which were widely interpreted as suggesting military action against IS could be justified by Christian theology.

On Tuesday, Francis said: "We have to fight terrorism. But when you have to stop an unjust aggressor, it has to be done with international consensus. No country can, on its own, stop an unjust aggressor."

Earlier in the day, the pontiff aimed a broadside at a Europe that "seems to give the impression of being somewhat elderly and haggard, feeling less and less a protagonist".

Francis - whose four-hour trip was the shortest abroad by any pope - was critical of the growing nationalist and anti-immigration sentiment that has surged in Europe amid economic stagnation and unemployment.