Rifts laid bare as G20 leaders meet
Mr Putin's spokesman, Dmitry Peskov, said Moscow regretted the decision. But he said: "If this is so, the president will have a couple of extra hours in the programme for useful meetings on the sidelines of the summit."
German Chancellor Angela Merkel has blamed the crisis "entirely" on Russia and said she would raise the issue with Mr Putin.
The second-highest profile meeting will also involve Mr Trump. He will have dinner on Saturday with his Chinese counterpart, Xi Jinping.
The key issue there will be the nations' trade tariffs dispute.
The US has hit a total of $250bn (£196bn) of Chinese goods with tariffs since July, and China has retaliated by imposing duties on $110bn of US products.
China said consensus was increasing on a deal but that differences remained.
After earlier indicating pessimism on an agreement, Mr Trump said: "They want to and we'd like to. There are some good signs. We'll see what happens."
Mr Putin has also weighed in, with a thinly veiled attack on Mr Trump's America First policy. The Russian president condemned "dishonest competition" along with "vicious" sanctions and protectionist measures.
The Brics group, comprising Brazil, Russia, India, China and South Africa, issued a statement saying protectionism ran counter to the "spirit and rules of the World Trade Organization".
What has been achieved so far?
Ahead of the summit's official start Mr Trump signed a trade deal with the Mexican and Canadian leaders.
Mr Trump described the US-Mexico-Canada Agreement (USMCA) - which replaces the Nafta free trade deal - as "probably the greatest trade deal ever".
"All of our countries will benefit greatly," he said.
Will there be a joint G20 communiqué?
European diplomats told Associated Press there has been tough haggling over a final statement.
Argentine officials indicated differences on trade could be overcome.
However, that will still leave the obstacle of climate change.
Mr Trump has not hidden his objection to collective action on the issue.
UN chief Antonio Guterres said in Buenos Aires that "this is a make-it-or-break-it moment" on the issue.
President Macron was quoted by AFP news agency as saying he would refuse to sign a trade deal with South America's Mercosur bloc if Brazilian President-elect Jair Bolsonaro withdrew from the Paris climate accord.
