Video report by ITV News correspondent Dan Rivers
Theresa May used a meeting with the Saudi Crown Prince at the G20 summit to "encourage" him to make sure Saudi Arabia co-operates in the investigation into murdered journalist Jamal Khashoggi.
The Prime Minister's stony-faced meeting with Mohammed bin Salman was in stark contrast to the friendly "high five" he shared with Vladimir Putin.
The two men greeted each other as if they were old friends, beaming and embracing.
The meetings took place at the G20 summit in Buenos Aires, Argentina - a meeting clouded by concerns over the gruesome killing of the Saudi journalist.
French President Emmanuel Macron also stepped up pressure on Saudi Arabia, telling bin Salman international experts need to be involved in the investigation into the murder.
A spokesperson for the Prime Minister said she "stressed the importance of ensuring that those responsible for the appalling murder of Jamal Khashoggi are held to account, and that Saudi Arabia takes action to build confidence that such a deplorable incident could not happen again.
"Noting the steps taken by the Saudi investigation since the Foreign Secretary had met with the Crown Prince and King Salman on November 12, she encouraged the Crown Prince to ensure that Saudi Arabia co-operated fully with the Turkish authorities and worked to bring both investigations to an acceptable close."
The Crown Prince was filmed in an apparently tense conversation with Emmanuel Macron, telling the French president not to worry.
Mr Macron said: “I am worried…You never listen to me.”
The widely-shared video showed bin Salman replying: “I will listen, of course.” It concluded with Mr Macron saying: “I am a man of my word.”
Later, an Elysee Palace official said the president had conveyed a “very firm” message on the need for international involvement in the Khashoggi investigation, currently the subject of two separate probes by Saudi Arabia and Turkey.
US intelligence agencies have concluded the crown prince ordered Khashoggi's killing - Saudi Arabia has denied that.
It's the first time the forum has met in South America.
Here's what you need to know about the G20.
What is the G20?
The Group of Twenty is a forum which sees 19 leading countries and the European Union meet annually to discuss policies to address the world’s “most pressing challenges”.
The member countries are Argentina, Australia, Brazil, Canada, China, France, Germany, India, Indonesia, Italy, Japan, Mexico, Russia, Saudi Arabia, South Africa, South Korea, Turkey, the United Kingdom, and the United States.
The 19 members and the EU account for around two in three of the global population and nearly 80% of world trade
Who else is invited?
Previous summits have seen participants who are not permanent members of the G20 invited, and Spain is a permanent invited guest and will be represented in Argentina.
The summit’s hosts have also invited Chile and the Netherlands, while several partner organisations such as the African Union, Association of Southeast Asian Nations, International Monetary Fund, United Nations, World Bank, World Health Organisation and the World Trade Organisation are typically present.
What is the history of the G20?
The G7 is a group of the largest advanced economies in the world and comprises Canada, France, Germany, Italy, Japan, the United Kingdom and the United States. Russia was previously a member of the then G8 but was ejected over the annexation of Crimea.
The G7 has a stronger focus on politics in industrialised countries, including discussions on health, energy, environment and terrorism, while the G20 looks at economic issues facing developed and emerging economies.
What is the difference between the G7 and G20?
The idea for the G20 started within the G7 meeting of finance ministers in 1999 who felt they needed a more broad group to address the world’s financial challenges.
After the financial crisis of 2008, the G20 worked on stabilising the world’s economy and also looks at markets, trade and development.
What will be the focus of the G20 summit in Argentina?
President Mauricio Macri said the slogan of the summit is a “fair and sustainable future” with a focus on “putting people first” and “building consensus”.
Discussions at the event take place behind closed doors, but the aim is to create a final document which all members agree on to promote “fair and sustainable development”.