Saudi Arabia's Oil Minister Khalid al-Falih talks to journalists at the beginning of an OPEC meeting in Vienna, Austria, June 22, 2018. REUTERS/Heinz-Peter Bader

VIENNA (Reuters) - OPEC agreed on Friday to raise oil production by around 1 million barrels per day from July for the group and its allies, an OPEC source said.

The output gain is nominal. The real increase will be smaller because several countries that recently underproduced oil will struggle to return to full quotas while other producers will not be allowed to fill the gap, OPEC sources have said.