Boris Johnson has said he fears there is only a 60% chance that the $100bn in climate finance viewed as key to securing an ambitious outcome to the Cop26 summit will be in place by the time world leaders meet in Glasgow in November.

Speaking to journalists en route to New York at the start of a three-day visit to the US, in which he hopes to “galvanise” progress towards a new climate deal, the prime minister said he would be urging developed countries to come forward with additional funding.

“There’s a roundtable with the climate-vulnerable countries [on Monday]: they need support from the rest of the world if they’re going to make the transition in the way that they must. And the developing world looks to us,” he said.

“We began fossil fuel emissions, it was our country that had the first sustained Industrial Revolution. We began it, and they look to us to help them move beyond hydrocarbon technology.”

Asked if he thought the $100bn total could be reached this week, he said: “Getting it all this week is going to be a stretch. Getting it all done by Cop, six out of 10. It’s going to be tough.”

G7 countries promised more than a decade ago at the Copenhagen summit to make $100bn in private and public finance available to developing countries, to help them transition to low-carbon technologies and cope with extreme weather.

Analysis from the Organisation for Economic Co-operation and Development (OECD) published last week suggested that so far only $80bn has been raised, however.

G7 leaders recommitted to the target at June’s summit in Cornwall, but Johnson was criticised for failing to secure concrete pledges. The UK is expected to publish a breakdown of individual countries’ commitments at the end of this week.

Cop26 chair Alok Sharma travelled with Johnson to the US, as the UK steps up diplomatic efforts to secure a successful outcome to the summit.

Johnson will hold a string of bilateral meetings with fellow leaders, and said he would be pressing them about “coal, cars, trees and cash”.

“We’ll be pushing for everybody to step up to the plate on getting rid of coal-fired power-stations, in the way that we have in the UK – a massive reduction in our dependence on coal; getting rid of hydrocarbon internal combustion engine motor vehicles, so that we all move towards electric vehicles; and planting hundreds of millions of hectares of trees; and getting the finance that’s necessary to do this,” he said.

The government has announced that £550m of the £11.6bn it had set aside for climate finance over the next five years will be allocated to developing countries, as part of the UK’s contribution to the $100bn target.

There are growing concerns about China’s attitude to the talks, after Beijing was irked by the Aukus deal – the security and defence pact between Australia, the UK and the US. Speaking on Sunday, Sharma was unable to say whether President Xi would attend Cop26.

Johnson lavished praise on China’s role in tackling the climate crisis, saying: “The Chinese actually have stepped up. They’ve gone a long way already and I congratulate President Xi on his vision.”

He added, “Alok has had some great conversations already with his Chinese counterparts about the things they want to do. I think China is massively important on this but it shows real signs of making progress.”

The prime minister will meet Brazilian president Jair Bolsonaro on the sidelines of UN general assembly this week. Asked whether he would challenge the avowed climate sceptic about the burning of the Amazon rainforest, Johnson said: “Yes. We want to stop and reverse the global loss of biodiversity, including in the rainforest.

“I believe it is in the long-term economic interest of all rainforest countries to do that. We want to plant gazillions of trees, hundreds of millions of hectares.

“We want a global process of reforestation. I think it is in the long-term interests of Brazil and the people of Brazil to recognise the spectacular natural endowment they have and to conserve it, and I am sure that President Bolsanaro agrees with that.”