By Crispian Balmer and Jan Strupczewski

ROME (Reuters) -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 their pledges on climate action, while failing to set tough new targets that activists say are vital to prevent environmental catastrophe.

Italian Prime Minister Mario Draghi greeted leaders from an array of countries, including U.S. President Joe Biden and Indian Prime Minister Narendra Modi, for the first face-to-face G20 summit in two years as the COVID-19 pandemic starts to ebb.

However, the Chinese and Russian presidents stayed away because of their continued concerns over COVID, dimming hopes of major progress in climate diplomacy ahead of the forthcoming COP26 summit in Glasgow, which is seen as vital to tackling the threat of rising temperatures.

British Prime Minister Boris Johnson acknowledged the talks in Rome and Glasgow 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.

The draft of the final communique said G20 countries, which account for up to 80% of the world's carbon emissions, 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 statement also said the leaders recognised "the key relevance" of achieving net zero carbon emissions by the middle of this century - a goal some of the world's largest polluters have still not committed to.

While the climate debate will dominate in Rome, much of the first day of the summit, which is being held in a futuristic convention centre called "The Cloud", will be given over to discussing the COVID-19 health crisis and economic recovery.

Fears over rising energy prices and stretched supply chains will be addressed. Leaders were also expected to endorse plans to vaccinate 70% of the world's population against COVID-19 by mid-2022 and create a task force to fight future pandemics.

Biden will urge the major 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 told reporters.

Biden's hopes of showing that his country was at the forefront of the fight against global warming took a knock after he failed to convince fellow Democrats this week to unify behind a $1.85 trillion economic and environmental spending package.

However, John Morton, the top climate adviser at the U.S. Treasury, said the fact that climate had vaulted to the top of the G20 agenda marked a remarkable shift.

"Obviously, this administration has come back in guns blazing on the issue in really important ways." he told Reuters.

There was also expected to be a lot of diplomacy on the sidelines in Rome, with numerous bilateral meetings planned, while the leaders of the United States, Britain, Germany and France were due to hold four-way talks on Iran.

Rome has been put on high security alert, 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.

(Additional reporting by Andrea Shalal, Angelo Amante, Jan Strupczewski and Gavin Jones Editing by David Gregorio and Helen Popper)