The heads of the world's 20 biggest economies kicked off two days of talks on Saturday where they were set to acknowledge the existential threat of climate change, but stop short of radical new commitments to tame global warming.

A draft communique seen by Reuters shows major countries are only likely to slightly toughen previous pledges on climate action, while failing to set tough new targets that activists say are vital to prevent environmental catastrophe.

However, more progress was made on other fronts, with the leaders endorsing a minimum tax agreement that all countries can collect from corporations from 2023 - a landmark deal aimed at stopping big business from parking profits in tax havens.

Leaders were also expected to back plans to vaccinate 70 percent of the world's population against Covid-19 by mid-2022 and create a task force to fight future pandemics.

"From the pandemic, to climate change, to fair and equitable taxation, going it alone is simply not an option," Italian Prime Minister Mario Draghi told the start of the meeting being held in a glass and steel conference centre, known as "The Cloud".

The G20 bloc, which includes Brazil, China, India, Germany and the United States, accounts for more than 80 percent of the world's gross domestic product, 60 percent of its population and an estimated 80 percent of global greenhouse gas emissions.

Many of the G20 leaders, including U.S. President Joe Biden, will fly straight to Glasgow for the start on Monday of the United Nation's climate summit, known as COP26, which is seen as crucial to addressing the threat of rising temperatures.

Hopes of making major progress in Rome were dimmed by the decision of Chinese President Xi Jinping and Russia's Vladimir Putin to stay at home and only follow events via video.

British Prime Minister Boris Johnson acknowledged the G20 and COP26 talks would be difficult, but warned that without courageous action, world civilisation could collapse as swiftly as the ancient Roman empire, ushering in a new Dark Age.

"It's going to be very, very tough to get the agreement we need," he told reporters early on Saturday.

Thin on details

The draft of the final communique said G20 countries will step up their efforts to limit global warming at 1.5 degrees Celsius - the level scientists have said is necessary to avoid disastrous new climate patterns.

The document also acknowledges that current national plans on how to curb harmful emissions will have to be strengthened, but offered little detail on how this should be done.

Additionally, the leaders are set to pledge to halt financing of overseas coal-fired power generation by the end of this year, and to "do our utmost" to stop building new coal power plants before the end of the 2030s.

While the climate debate will dominate in Rome, much of the first day of talks were given over to discussing the Covid-19 health crisis and economic recovery.

Fears over rising energy prices and stretched supply chains will be addressed, while Biden was expected to urge G20 energy producers with spare capacity to boost production, notably Russia and Saudi Arabia, to ensure a stronger global economic recovery, a senior U.S. administration official said.

The U.S. president was later due to hold talks with the leaders of Britain, Germany and France on Iran's nuclear ambitions - just one of numerous meetings being held on the sidelines as the G20 chiefs catch up on in-person diplomacy.

"It is great to see all of you here, after a difficult few years for the global community," Draghi said.

Rome has been put on high security alert from the weekend, with up to 6,000 police and about 500 soldiers deployed to maintain order.

Two protest rallies have been authorised during the day, but demonstrators will be kept far from the summit centre, located in a suburb built by the 20th Century fascist dictator Benito Mussolini.