Brazilian President Luiz Inacio Lula da Silva called for South American unity as he hosted fellow leaders for a regional "retreat," but drew barbs for his warm welcome of Venezuelan socialist Nicolas Maduro.

Veteran leftist Lula, who returned to office in January after leading Brazil from 2003 to 2010, is looking to strengthen diplomatic ties in a region where left-wing governments are newly back in style.

But he faced criticism for hosting Maduro, a pariah in some quarters for his government's alleged human-rights violations and crackdown on political dissent – a depiction Lula questioned on Monday as a hostile "narrative".

The issue exposed fissures at what was meant to be a display of South American diplomatic goodwill and cooperation.

"I was surprised to hear what's happening in Venezuela described as a 'narrative,'" said Uruguay's centre-right President Luis Lacalle Pou, who has labelled Maduro a "dictator".

"The worst thing we can do is try to sweep that under the rug," he told the summit.

"Human rights must be respected everywhere, always, no matter the political colors of the leader in power," said Chile's left-wing President Gabriel Boric.

However, Boric backed the Venezuelan government's call for Washington and the European Union to lift sanctions on Maduro and his inner circle.

Maduro responded by saying Chile and Uruguay "have one vision" and Venezuela, "another".

"The most important thing is that there has been a debate," he said, announcing "a new stage" of South American integration.

Lula defended Maduro, whose country he said was experiencing a period of "tranquility".

"The same demands that the democratic world makes for Venezuela it does not make for Saudi Arabia," Lula said later at a press conference.

He had warmly welcomed Maduro to Brasilia Monday, reversing the policy of his far-right predecessor, Jair Bolsonaro, who had cut ties with Venezuela's socialist government and joined a US-led group of more than 50 countries in recognising opposition leader Juan Guaido instead.

Lula, who greeted Maduro with a hug, hailed it as a "new moment" in the countries' relations. (AFP)