PM says she wants to see ‘those responsible being held to account’ as she juggles fraught diplomacy with shoring up Brexit deal on world stage

Theresa May has said en route to the G20 summit that she plans to raise the killing of Jamal Khashoggi and the situation in Yemen with Saudi Arabia’s Crown Prince Mohammed bin Salman.

“I am intending to speak with the crown prince of Saudi Arabia. The message that I give will be very clear ... on this issue of Jamal Khashoggi but also on the issue of Yemen,” the prime minister told reporters before touching down in Buenos Aires. “[On Khashoggi’s killing] we want to see a full and transparent investigation in relation to what happened and obviously those responsible being held to account.

“On the issue of Yemen we continue to be deeply concerned about the humanitarian situation,” May said. “The long-term solution for the Yemen is a political situation and we will be encouraging all parties actually to look for that and work for that.” Western nations are calling for an end to the Saudi-led military campaign in Yemen launched by Salman.

May has flown to Buenos Aires hoping to rescue the international reputation of her Brexit deal as she navigates a fraught diplomatic atmosphere with Saudi and Russian leaders. During the two-day summit in Buenos Aires she will hold up to six bilateral meetings with world leaders, but not with Donald Trump, who has been critical of her Brexit deal and its potential limitations on future trade.

Nor will she meet Vladimir Putin, given the frosty state of UK relations with Moscow and division among G20 leaders over how to regard Russia’s latest incursions in Ukraine.

During the flight to the summit, May said the latest incident was part of a pattern of Russian behaviour” and the UK would continue to call for sanctions. She added: “If you look at the message I gave at my Mansion House speech, Russia could take a different road, Russia could support the rules-based international system. And if it did, its position would be a different one. It is choosing to continue with this pattern of behaviour.”

In Saudi Arabia, diplomatic relations have been testy with Salman over the killing of Khashoggi and the escalation of suffering in the war in Yemen. Downing Street sources suggested the pair would be “engaging” without officially confirming a bilateral meeting.

“We have been engaging with the Saudi government in recent weeks in the wake of the murder of Mr Khashoggi and we have been doing so in order to deliver a message that we want full accountability and full transparency,” a senior UK official said before May’s departure.

“If the opportunity arises for the PM to repeat that message and deliver important messages in relation to Yemen, for instance, then I’m sure she will take the opportunity to do so. Engagement is important if we are to address these issues.”

May’s arrival on Thursday night marked the first visit by a British prime minister to the Argentinian capital. On Friday she will tell world leaders she has achieved a good Brexit deal for the global economy that will open up new opportunities for trade and give the UK a new, independent voice to push for reform of multinational institutions such as the World Trade Organization.

Trump has expressed deep scepticism about the UK’s ability to strike free-trade deals with countries such as the US under the agreed Brexit terms, a fear shared by many of May’s mutinous backbenchers seeking to crush her deal in parliament.

Downing Street hit back after Trump made his remarks on Monday, insisting the UK could negotiate, sign and ratify agreements provided these did not enter into force or apply during the Brexit transition period.

May’s pre-summit remarks made no mention of the deal’s uncertain future. “The Brexit deal we have agreed sets a path for the UK to a brighter future,” she said, adding that the UK was already taking “significant steps to boost trade in fast-growing new markets”.

May will use the summit to underline that although the UK will remain close to the EU, she does not believe the UK will be restricted in signing new trade deals.

“For the first time in more than four decades, the UK will have an independent trade policy. We will play a full and active role on trade on the global stage, working with friends new and old at a time of unprecedented global interconnectedness,” she said.

The prime minister took a similar tone during a 90-minute grilling by parliament’s liaison committee before her flight, batting away repeated questions about the risks of a no-deal departure by saying this placed greater responsibility on MPs to endorse her plan.

The government was preparing for the possibility of no deal, May said: “But of course the way to ensure that we get a good deal and we are able to see that smooth and orderly exit is to ratify the deal that the government has agreed.”

The prime minister also argued that extending the article 50 timetable to potentially allow time for a second referendum would mean having to reopen talks with the EU on the planned deal – a view dismissed by officials in Brussels.

One of the new freedoms that May will promote at the summit will be the UK’s independent seat at the WTO from April 2019, which Downing Street said would be used to push for “ambitious and urgent reform” including on trade in services, declaration of subsidies and respect for intellectual property rights.

Downing Street’s promotion of the prospect of free-trade deals was somewhat undermined by the cross-Whitehall economic analysis published on Wednesday, which suggests trade deals with the US, Australia and Brics countries would add just 0.2% to GDP.

A UK government official said there were limitations to the analysis. “Any growth we can improve in GDP is a positive … the global economy is changing and the fact we can have our own independent trade policy puts us in a good place to make the most of those opportunities,” the official said.

May’s first bilateral talks will be with the Argentinian president, Mauricio Macri, where she is expected to announce the first UK trade envoy to the country – an honour usually bestowed on an MP or peer – who has not yet been confirmed.

A Downing Street source said the prime minister expected Macri would raise the issue of the Falkland Islands but played down the prospect of any significant developments, and said May would restate the UK’s longstanding position. May will not visit the Falkland Islands on this trip.

“It’s Argentina’s G20 and trade is clearly the priority,” the UK official said. “If you look broadly at the relationship that we have with President Macri … it does suggest the relationship is moving in a positive direction.”