German Chancellor Angela Merkel, who is hosting the summit in Hamburg, said the talks so far had been “very difficult”.
Negotiators worked through the night in an attempt to reach a compromise on the wording of the final summit statement.
It comes after a second night of violent protests on Hamburg’s streets.
The G20 (Group of Twenty) is a summit for 19 countries, both developed and developing, plus the EU.
Leaders at the summit are struggling to find common ground with the US following President Donald Trump’s decision to withdraw from the Paris climate agreement last month.
A separate issue is Mr Trump’s rejection of free trade – although the president has said that he is not opposed to trade in principle, his position is that any trade deals agreed have to protect US industry.
Mrs Merkel said: “The discussions are very difficult, I don’t want to talk around that.”
She added that she hoped that the differences with the US would not affect the commitments made by other nations.
Her comments came amid large protests on the streets of Hamburg, with demonstrators and heavily armed police clashing into the early hours on Saturday.
Demonstrators – who were protesting against the presence of Mr Trump and Mr Putin, climate change and global wealth inequalities – set fire to vehicles and barricades, threw rocks at officers and looted shops.
At one point, police chased protesters across rooftops while officers on the streets used water cannon on protesters.
Nearly 200 police officers were injured during the protests.
German military officers were later despatched to help bring order to Hamburg’s streets.
Dozens of protesters have been detained by police.
Mrs Merkel said she could understand peaceful protests, but said that demonstrations that “put lives in danger” were “unacceptable”.