BUENOS AIRES, Argentina — World leaders arrived Thursday in the Argentine capital for the Group of 20 summit of the globe's largest economies as issues such as a trade war between the United States and China, the killing of a Saudi journalist in the country's Istanbul Consulate and the conflict over Ukraine threatened to overshadow the gathering.

The two-day summit beginning Friday is supposed to focus on development, infrastructure and food security, but those seemed largely an afterthought amid soured U.S.-European relations and as the United States, Mexico and Canada hammered out the final language of a replacement for the North American Free Trade Agreement expected to be signed Friday.

Michael Shifter, head of the Inter-American Dialogue, a Washington-based think tank, said that this G-20 summit was once considered an opportunity for Latin American members Argentina, Brazil and Mexico "to project a regional bloc to shape a global agenda."

But, he said, "that turned out to be a fleeting aspiration."

"The fact that the G-20 is taking place in South America for the first time is almost beside the point," Shifter said. "Argentine President Mauricio Macri, the summit's host, has lowered expectations. ... Now a success would be a summit meeting that goes smoothly, without any major disruption."

Nonetheless, French President Emmanuel Macron, who flew into Buenos Aires on Wednesday as one of the earliest arrivers, clung to the importance of the ideal of cooperation that the G-20 represents.

"I believe in our capacity to make the spirit of dialogue and cooperation triumph," Macron said at a joint news conference with Macri, warning that if nations "close down," the alternative could be trade wars or armed conflict.

Macron also called for international involvement and "complete clarity" in investigations into the killing of dissident Saudi journalist Jamal Khashoggi, and said European leaders should discuss it at a meeting Friday.

Macri said the matter of the killing would be "on the table" during bilateral and possibly broader meetings.

Saudia Arabia has denied that Crown Prince Mohammed bin Salman played a role in Khashoggi's gruesome slaying. But Human Rights Watch accuses him of responsibility and also of war crimes in Yemen, and on Wednesday, Argentine legal authorities took initial action to consider a request to prosecute him for alleged crimes against humanity, a move apparently aimed at embarrassing him as he attends the summit.

It is to be bin Salman's first significant appearance overseas since the killing. Turkey's Recep Tayyip Erdogan, who has been sharply critical of Saudi Arabia over the incident, is also in attendance.

"Given the role that Turkey has played in this, given that the murder happened at the Saudi consulate in Istanbul, this will be an interesting meeting," said Willis Sparks, director of global macro politics at Eurasia Group. "Just to see how leaders interact with the crown prince will be interesting — how warm they are. I expect (U.S. President Donald) Trump to be very warm with him, but European leaders probably are going to be very reluctant to have their pictures taken with him."

An expected high-profile bilateral meeting between Trump and Russia's Vladimir Putin planned for Saturday was abruptly canceled by Trump, who made the announcement in a tweet citing Russia's seizure of Ukrainian vessels over the weekend.

The Kremlin said it had not been notified and only learned about it from the tweet. Russian news agencies quoted Putin spokesman Dmitry Peskov as saying the cancellation means that Putin will have "a couple more hours" for "useful meetings" with G-20 leaders.

Trump was still scheduled to meet with Chinese President Xi Jinping, but analysts were not optimistic about prospects for a major breakthrough on the two countries' trade disputes a month before U.S. tariffs on Chinese goods are set to ramp up.

Shannon O'Neil, an expert on global trade at the Council on Foreign Relations, said she believes it "very likely" that the tariffs will take effect in January.

"I think this is an issue that Trump cares a lot about and is going to use when he campaigns for 2020," O'Neil said. "It used to be Mexico and NAFTA, and now it's going to be China."

Trump, Canadian Prime Minister Justin Trudeau and Mexican President Enrique Pena Nieto are scheduled to sign the new United States-Mexico-Canada Agreement that is replacing the NAFTA trade deal during a ceremony on Friday.

Canadian Foreign Minister Chrystia Freeland said in a video posted online by Toronto's CityNews that a "vast number of technical details need to be scrubbed and wrapped up," but the three countries are "very much on track" to sign on time.

The pact must still be approved by lawmakers in all three countries. O'Neil said she anticipates that to be "quite smooth" in Mexico and Canada, but passage could be more complicated in the United States after midterm elections flipped the House of Representatives, meaning the next speaker could be Democratic Rep. Nancy Pelosi.

"There are some ... things in there that I think Democrats can support," O'Neil said. "But I can't imagine having a new NAFTA is going to be Pelosi's first priority when she comes in, so I'd expect it to be drawn out."

It stands to be a short visit for Pena Nieto, who is scheduled to return to his country for the inauguration Saturday of Mexican President-elect Andres Manuel Lopez Obrador.

Also Thursday, Macron criticized protectionist stances by Trump but said they have no plans for a one-on-one at the summit. The two have increasingly clashed in recent weeks on everything from Trump's nationalism to wine tariffs.

The French president envisions himself as a new leader of the free world and is fashioning himself at this summit as the anti-Trump — a champion of the Paris climate accord, defender of the postwar system of global trade and crusader against multinational tax evaders.

Macron warned that Europe might not sign trade deals with the South American regional bloc Mercosur if Brazil's incoming president, Jair Bolsonaro, pulls out of the Paris accord.

Taking the world stage at the G-20 is a welcome relief for Macron, who has faced mass protests at home over rising fuel taxes that are the biggest challenge yet to his presidency. But his party dominates parliament and neither faces re-election until 2022.

Other European leaders at the summit are facing domestic struggles of their own. Britain's Theresa May is fighting for political survival as she tries to pull her country out of the European Union. Germany's Angela Merkel is preparing to leave politics after announcing last month she would give up leadership of her party, a post she has held since 2000. Italy's Giuseppe Conte heads a populist coalition that is clashing with the EU and suffers internal divisions.

Senior German officials, briefing reporters in Berlin on condition of anonymity, said Merkel planned to hold bilateral meetings with Trump, Putin, Xi, India's Narendra Modi, Australia's Scott Morrison and Macri. Merkel was supposed to arrive in Buenos Aires early Friday, but her plane returned to Germany on Thursday night due to a technical problem. A German air force plane was being readied to carry the chancellor and her entourage.

The British Embassy in Argentina said May's visit would be the first by a U.K. prime minister to Buenos Aires; the only other prime minister to visit the country was Tony Blair who went to Puerto Iguazu in 2001. The two countries have long been at odds over the disputed islands known as the Falklands in Britain and the Malvinas in Argentina.

Outside Argentina's congress, as many as 1,000 people gathered Thursday for a forum hosted by organizations opposing the G-20 and the International Monetary Fund. A large inflatable blimp caricaturing Trump as a baby holding a cellphone — which has appeared at protests in other places the U.S. president has visited — floated over the square underneath a light rain.

Thomas Bernes of the Centre for International Governance Innovation, a Canada-based think tank focusing on global governance, said this summit could be a defining moment for the Group of 20 — for better or for worse.

"The G-20 Leader's Summit is at risk of falling into disarray with the summit being overshadowed by items not on agenda," Bernes said. "The true test will be whether the other members of the G-20 will act resolutely or whether we will witness the crumbling of the G-20 as a forum for international economic cooperation."