Francis, the first non-European pope in 1,300 years, said he was afraid of "very dangerous alliances among (foreign) powers that have a distorted vision of the world: America and Russia, China and North Korea, (Vladimir) Putin and (Bashar al-)Assad in the war in Syria".
He said the greatest danger concerned immigration, with "the poor, the weak, the excluded and the marginalised" juxtaposed with "those who... fear the invasion of migrants".
European Union states are at odds over how to cope with a huge influx of migrants, many fleeing war and poverty in countries Syria, Afghanistan and other countries.
On top of resolving the differences over trade and climate change Angela Merkel, chancellor of G20 host nation Germany, is expected to lead discussions on this issue.
Pope Francis was also quoted as saying Europe should adopt a federal structure as soon as possible or "it won't count for anything in the world"
In May the 79-year old Argentine urged Europe not to see migrants as criminals.