The so-called Summit of the Americas is set to be hosted by the United States in early June, marking the ninth meeting of countries in the region and the first time the US has hosted the gathering since 1994.

But the lead up has already been embroiled in controversy over the guest list, forcing US officials to try to smooth over relations and throwing into question the outcome of the meeting at a critical time in the hemisphere.

Earlier this month, Mexican President Andrés Manuel López Obrador said Mexico's participation will not be confirmed until the US invites every country in the hemisphere, arguing that no country should be excluded from the summit. US officials have repeatedly said the governments of Cuba, Nicaragua, and Venezuela will not be invited to the summit due to their human rights records.

But in recent days, the administration has reversed some Trump policies related to Cuba and eased some energy sanctions on Venezuela, signaling the importance of attendance at the summit -- and the importance of avoiding an embarrassing boycott by key countries at a tough political moment for President Joe Biden.

"We are in dialogue, with the purpose of inviting everyone," López Obrador said at a news conference Monday. "At least, they (United States) have acted in a respectful manner, there has not been a total, cutting rejection."

"There are still days to go, I hope that this week we will be able to inform, so as not to be speculating, or with conjectures, leaks; once we have all the elements, we are going to establish our position here," Lopez Obrador added.

An administration official told CNN the Biden administration is evaluating options on incorporating "the voices of the Cuban, Venezuelan, and Nicaraguan people into the Summit process."

If López Obrador skips the gathering and others follow, it would be a snub to the Biden administration, which has stressed relationships with Latin America and has sought to strengthen ties as China makes inroads in the region.

Last week, the US and Mexico held talks about options specific to Mexico for attending the summit, according to a source familiar with the discussions. Talks are ongoing and a decision has not yet been made, the source said.

Former Sen. Christopher Dodd, who's serving as special adviser for the summit, led discussions for the US by phone last Wednesday, according to a White House official. The discussions, the official said, were wide-ranging and included talks about Mexico's attendance.

The first wave of invitations for the Summit of the Americas went out last week, according to the White House official, adding the administration is considering additional invites. The White House hasn't released the list of invitees.

The ongoing back-and-forth, though, has sown doubt among US partners in the region over the summit's effectiveness.

"It's a summit that is organized around conversations among presidents or prime ministers," a senior Guatemalan official told CNN. "The summit is important in itself that it happens, but it would be less of a success if it's not at the highest level."

Prior to the invitations being sent out, Guatemalan President Alejandro Giammattei said he wouldn't attend the summit after the United States criticized the reappointing of his nation's attorney general. It's unclear whether he'll change his position following the release of invites.

Latin American countries other than Mexico have taken issue with invites not being extended to Cuba, Venezuela and Nicaragua. Honduran President Xiomara Castro de Zelaya said on Twitter, "If not all nations are present, it's not a Summit of the Americas."

Guatemala and Honduras have been part of Vice President Kamala Harris' portfolio tackling root causes of migration. In late January, Harris also attended the presidential inauguration of Castro de Zelaya. But she has not reached out to either country in the lead up to the summit.

Argentinian President Alberto Fernandez, meanwhile, said he'll attend the summit but echoed concerns about excluding nations. It's unclear, though, whether Brazilian President Jair Bolsonaro -- one of the largest countries in the hemisphere -- will attend.

Bolivian President Luis Arce went a step further, saying he wouldn't participate if countries were excluded.

Matthew Rooney, director of Institute Outreach and Strategic Partnerships at the George W. Bush Presidential Center, noted that it's not out of character for the summit to only invite democratic leaders but the pushback of the last few weeks speaks to the shifting dynamics in the region.

"The United States should be able to invite who it wants to invite to its home and the other guests should be happy to be invited," Rooney told CNN. "It sends a political signal that the drawing power of the United States is not what it used to be and the force of the summit's commitment to democracy is not what it used to be."

The summit comes at a time of massive migration in the Western Hemisphere. According to the Department of Homeland Security, more than 6 million Venezuelan refugees and migrants have fled the country. Nicaraguans have also increasingly been migrants, as well as Haitians who had moved to the region years ago.

Elements of a migrant protection pact are being circulated and discussed among countries ahead of the summit, the senior Guatemalan official said.

First lady Jill Biden visited Ecuador, Costa Rica and Panama in recent days to "emphasize the importance of the US partnership" ahead of the June summit.

Upon departing her trip to Latin America, Biden answered a question as to whether she was reassured the countries she visited would be attending the Summit of the Americas.

"All of the countries I've visited said that they would be there. I'm looking forward to it. It's in like 10 days," she said.

Asked if she was concerned about threats of boycott from certain countries, she said, "I'm not worried. I think that they'll come."