The UK, the US and the EU are embarking on a frantic round of climate diplomacy in a last-ditch attempt to bring key countries into a deal on greenhouse gas emissions before the Cop26 climate summit.

Alok Sharma, the UK cabinet minister who will preside over the talks, has meetings planned with representatives of China after questions were raised over whether the president, Xi Jinping, would attend Cop26 in person, as well as the other G20 big emitters yet to produce plans on emission cuts before the summit, which opens in Glasgow on 31 October.

The European Commission vice-president, Frans Timmermans, is going to India and Indonesia, also G20 countries, this weekend. India is the world’s third biggest emitter of greenhouse gases, after China and the US, and Indonesia is heavily reliant on coal. John Kerry, the US climate envoy, is also meeting with representatives of the G20 countries China, Saudi Arabia and Mexico, and has met Russian officials in recent days.

The leaders of the G20 are scheduled to meet in Italy in the days before Cop26. G20 countries are responsible for about 80% of global emissions and are crucial to the UK’s hopes for a Cop26 outcome that will “keep 1.5C alive”.

Holding global temperature rises “well below” 2C above pre-industrial levels, and “pursuing efforts” to hold them to 1.5C, are the goals of the 2015 Paris climate agreement.

Kerry gave the most upbeat assessment yet of the talks recently, in an interview with the Guardian, when he said world leaders were “sharpening their pencils” to agree new commitments. The UK’s diplomats are also “cautiously optimistic” and the EU believes progress will be made on key issues.

More than 20 countries joined in a pledge to cut methane, a potent greenhouse gas, early this week. The UK believes a deal on providing $100bn to developing countries will be achieved, and progress is being made on a deal to halt deforestation and the destruction of nature.

Developing countries are also now focusing “laserlike” on the G20, as several major countries – chiefly China, Russia, India, Saudi Arabia and Brazil – have not yet come forward with the fresh commitments needed to make Cop26 a success.

At Cop26 nations are asked to come forward with new plans, called nationally determined contributions (NDCs), to cut emissions by 2030. However, all of the big players acknowledge that Cop26 will not produce strong enough NDCs from all countries to amount to the 45% cut by 2030 needed to stay within the 1.5C goal.

Instead, they believe substantial progress will be made on NDCs and in other areas – moves to phase out coal, halt deforestation and plant trees, phase out the internal combustion engine, and provide climate finance to developing countries – to keep the 1.5C goal in reach. Further progress on the core emissions cuts is expected to come next year and beyond.

The UK is also preparing to launch its own net zero strategy, which will be closely scrutinised by other countries, which need to see UK leadership in cutting emissions before making commitments themselves.

Concerns over China’s participation in the talks were played down by key figures after some news outlets reported Xi was unlikely to come. The Guardian understands that Xi’s participation in person is still possible, and the UN said China still intended to participate.

Kerry told the Guardian: “I am hopeful that President Xi is very much engaged and is personally making decisions, and personally committed.” He pointed to a long phone call between Xi and Biden recently in which the climate was discussed. “There was a very clear commitment to work with the US to achieve our goals. We are very hopeful.”

Although Xi may still decide to travel, analysts would not be surprising if he stayed away. The Chinese leader has not travelled abroad since January 2020, before Covid-19 became a pandemic. “I wouldn’t expect his first diplomatic engagement to be to the UK,” said Sam Geall, CEO of the London-based thinktank China Dialogue.

More recently, Xi did not attend the UN general assembly in New York in person, but delivered a speech via video link. He also did not travel from his office in Beijing to the southern Chinese city of Kunming for the Cop15 summit China has been hosting this week.

“Cop26 is not like signing the Paris agreement: there is no treaty being signed. The hope is to raise ambition, restore trust and faith in the process,” Geall said. “Xi made significant statements to the UN – last year with the 2060 [net zero] pledge, and this year with the coal phaseout pledge. China has set out their stall, made clear their long term trajectory.”

A developing country negotiator said China must step up to make Glasgow a success. “China’s leadership on climate action is critical. We can only [keep] the promise of the Paris agreement if we are in this fight together. And, without China at the table with us, there is no pathway to 1.5C. We remain encouraged by its commitment to halt overseas funding of coal, it’s a strong signal, and we will be looking to them for leadership in Glasgow on scaling up that ambition to achieve a successful outcome for all. We need China and all members of the G20 to step up ambition on both mitigation and adaptation with urgency.”

It is still not clear who the top official representing China would be if Xi does not travel. In recent international negotiations, Beijing’s veteran climate negotiator, Xie Zhenhua – now China’s special climate envoy – has been seen dealing with Kerry and Sharma. Analysts say that if Beijing were to bring extra “firepower”, the politburo standing committee member and vice-premier Han Zheng may be on his way to Glasgow, too.

In a boost to the talks, the Australian prime minister, Scott Morrison, on Friday announced he would attend, bringing the number of confirmed world leaders to 125.