Theresa May has vowed to maintain Britain's defence of the sovereignty of the Falkland Islands, as she became the first Conservative prime minister to visit Argentina since the 1982 war.
Mrs May will held one-on-one talks on Friday with Argentine president Mauricio Macri, who is hosting leaders from around the world at the G20 summit of major economies.
The Falklands, known by the Argentines as Las Malvinas, were expected to feature in their discussions, but were not expected to dominate talks which will focus on trade.
Mrs May faced potentially more awkward encounters in Buenos Aires with Saudi Arabia's Crown Prince Mohammed bin Salman in the wake the murder of journalist Jamal Khashoggi and Russia's Vladimir Putin following the poisoning of Sergei Skripal in Salisbury and the seizure of Ukrainian ships.
Mrs May held face-to-face bilateral talks with the crown prince on Friday afternoon, during which she raised the Khashoggi case as well as the Saudi-backed war on Houthi rebels in Yemen.
Mrs May said: "The message I give will be very clear. It is the message we have consistently given on this issue of Jamal Khashoggi and the issue of Yemen.
"In relation to Mr Khashoggi, we want to see a full and transparent investigation of what happened and those responsible being held to account.
"With the issue of Yemen, we continue to be deeply concerned about the humanitarian situation. The long-term solution for Yemen is a political solution, and we will be encouraging the parties to work for that political solution."
President Macri has toned down official rhetoric over the islands Falklands since taking over from Cristina Kirchner in 2015.
This week's announcement of a new air link between the islands and the South American mainland via Argentina has been greeted in London as an indication that relations are headed in a positive direction.
Asked during her flight to Buenos Aires whether the UK remained ready to defend the islands with military force if necessary, Mrs May said: "I am clear that our position on the sovereignty of the Falklands has not changed.
"What has changed in recent months is we have seen better relations with Argentina. I think the announcement we saw earlier this week of the extra flight from the Falklands through to South America is important.
"It is important for the Falklands and it is important in showing a different relationship developing.
"I am talking to President Macri about issues about trade and opportunities for trade, but our position on the sovereignty of the Falklands hasn't changed and will not change."
Mrs May is not expected to have bilateral talks with Mr Putin in Buenos Aires, as relations between the UK and Russia remain in the deep-freeze following the Salisbury poisoning and the annexation of Crimea.
Britain has been a keen supporter of sanctions against Moscow for its interference in Ukraine and Mrs May made clear she does not rule out their extension following the seizure of three naval ships and their crews in the Black Sea.
"Our message to Russia is very clear," she said.
"They should release the ships and sailors and deescalate the situation.
"If you look at what we have seen over the last year of so, this is part of a pattern of Russian behaviour.
"We have always been at the forefront in Europe of asking for sanctions on Russia in relation to its behaviour. We will continue to push for what we consider to be appropriate sanctions on Russia.
"Russia could take a different approach. Russia could support the rules-based international system. If it did that, its position would be a different one. It is choosing to continue with this pattern of behaviour."
The PM will use the G20 gathering to push her "global Britain" message, telling fellow leaders that her Brexit deal will be good for the world economy.
After resuming its independent seat on the World Trade Organisation in April next year, the UK will become an active and leading voice in seeking reform of the body to increase transparency and modernise its approach to e-commerce, she will say.
