LONDON: Forcing wealthy nations to honour their UN climate fund pledges this week will "be a stretch", British Prime Minister Boris Johnson admitted ahead of a Monday meeting with global leaders aiming to do just that.

At the 2009 Copenhagen climate conference, rich nations promised to provide annual funding of $100 billion from 2020, for poorer nations to counteract the effects of climate change.

The Organisation for Economic Co-operation and Development has said progress has been "disappointing", with developing countries receiving $79.6 billion in 2019.

World leaders return to UN with focus on pandemic, climate

"I think getting it all done this week is going to be a stretch," Johnson told reporters as he travelled to New York on Sunday to attend the UN General Assembly (UNGA), according to PA news agency.

He added that he saw the chances of getting it done before COP26 climate summit in Glasgow in November as "six out of 10".

"It's going to be tough but people need to understand that this is crucial for the world," he said.

He added that there were "real signs of progress" from China, the world's biggest emitter of carbon dioxide, even as COP26 president Alok Sharma said Sunday that Chinese President Xi Jinping had not yet confirmed his attendance at the conference.

Johnson will convene Monday's meeting of leaders along with UN Secretary-General Antonio Guterres.

"We only have a short time left. World leaders must deliver on their climate commitments ahead of @COP26," Johnson said in a tweet.

The UN climate fund is the main financing tool for the Paris Agreement, which calls for capping the rise in global temperatures at "well below" two degrees Celsius, and 1.5 C if possible.

The treaty calls for billions of dollars annually for poor nations already coping with floods, heatwaves, rising seas and superstorms made worse by climate change.

As well attending the UNGA, Johnson will visit the White House on his trip, days after the announcement of a new US-Australia-Britain security pact caused a deep rift with ally France.

He will also meet Brazilian President Jair Bolsonaro, known for his scepticism on climate change, and Amazon founder Jeff Bezos.

Asked if he would challenge Bezos on how much tax Amazon pays in the United Kingdom and workers rights, Johnson said he "certainly" would.

"But I will also be congratulating him on his massive forestry initiative. He's putting a huge amount into planting trees around the world," he said.