The West and its allies need to be united against authoritarianism, UK Foreign Secretary Liz Truss said on Saturday, as she hosted G7 counterparts with concern about threats from Russia and China.

The two-day gathering of foreign ministers from the world's richest nations in Liverpool, northwest England, is the last in-person gathering of Britain's year-long G7 presidency, before it hands over the baton to Germany.

Russia's build-up of troops on Ukraine's border is top of the agenda, alongside discussions on confronting China, limiting Iran's nuclear ambitions and addressing the crisis in military-ruled Myanmar.

"We need to come together strongly to stand up to aggressors who are seeking to limit the bounds of freedom and democracy," Truss said as she formally opened the talks, without mentioning specific countries.

"To do this, we need to have a fully united voice. We need to expand our economic and security posture around the world."

Truss held talks on the sidelines of the summit on Friday with US Secretary of State Antony Blinken as well as Germany's new Foreign Minister Annalena Baerbock.

Blinken flies on to Southeast Asia next week on a visit designed to highlight the region's importance in Washington's strategy of standing up to an increasingly assertive China in the region.

Ministers from the Association of Southeast Asian Nations (ASEAN) will join the G7 summit for the first time ever on Sunday, in a session earmarked for wide-ranging talks on issues including Covid-19 vaccines, finance and gender equality.

South Korea, Australia, South Africa and India will also participate as Britain's chosen G7 "guests", with many attendees taking part virtually due to the pandemic and emergence of the Omicron variant.

Truss said before the meeting that she wanted deeper ties between G7 nations in trade, investment, technology and security "so we can defend and advance freedom and democracy across the world". 

"I will be pushing that point over the next few days," she added.

- 'Alternative' to adversaries -
Truss, who replaced Dominic Raab as Britain's top diplomat in September, delivered her first major foreign policy address Wednesday as crises loom around the world.

She warned Moscow it would be "a strategic mistake" to invade Ukraine, following growing concerns over a big Russian troop build-up on the border.

That echoed comments delivered by US President Joe Biden to his Russian counterpart Vladimir Putin in a virtual summit the previous day.

Britain's G7 presidency has been dominated by responding to Beijing's increasing international assertiveness and alleged widespread domestic rights abuses, including of its Muslim minority Uyghur population.

This week, a panel of human rights lawyers and experts in London concluded Beijing had committed genocide by imposing population restrictions, including birth control and forced sterilisations, on the Uyghurs.

China rejected the tribunal's findings. 

At a G7 leaders' summit in June, Biden pushed for a stronger collective stance towards both China and Russia, and this week saw Washington, London and Canberra announce diplomatic boycotts of the 2022 Beijing Winter Olympics.

Truss has said the West needs to work towards ending the "strategic dependence" of a growing number of low- and middle-income countries on its adversaries, in various areas from energy to technology.

At the summit she will push attendees to provide those countries with more finance for infrastructure and technology projects, according to the foreign office.

G7 countries and their allies must offer "an alternative to unsustainable debt from non-market economies" like China, it said. 

Truss will unveil a UK-led initiative -- the Africa Resilience Investment Accelerator -- to boost collaboration investing in Africa's "most fragile markets" and help develop "a pipeline of investable opportunities".

"It will help the G7 to meet its commitment to invest over $80 billion into the private sector in Africa over the next five years to support sustainable economic recovery and growth," the foreign office added.