LOS ANGELES — President Biden on Thursday promised aggressive new leadership to confront economic despair and mass migration in Latin America, but his pledge was met with skepticism from the leaders in the region, who said the United States is doing too little to meet the grim moment of a global crisis.

In a speech to leaders gathered at the ninth Summit of the Americas, Mr. Biden urged those in attendance to join together behind concrete commitments to deal with poverty, corruption, violence and climate change. He vowed to demonstrate America’s “enduring investment in our shared future.”

“There is no reason why the Western Hemisphere can’t be the most forward-looking, most democratic, most prosperous, most peaceful, secure region in the world,” the president told the gathering.

But some of Mr. Biden’s counterparts viewed what he called “strong and constructive diplomacy” with less optimism.

A boycott of the summit by the leaders of several Latin American nations — including Mexico, a regional powerhouse — continued to undermine the gathering organized by the United States.

Mr. Biden had hoped to assemble the leaders of the hemisphere as a show of American strength and unity of purpose. Instead, his refusal to invite several authoritarian leaders prompted the leaders of Mexico, Guatemala, Honduras, Bolivia and El Salvador to refuse to attend.

Johnny Briceño, the prime minister of Belize, publicly chastised Mr. Biden in a remarkable speech just moments after the president had left the lectern. Mr. Briceño said it was “inexcusable” that the United States had blocked Cuba and Venezuela from attending the summit, a decision that sparked the boycott by four countries.

“At this most critical juncture, when the future of our hemisphere is at stake, we stand divided,” he said. “And that is why the Summit of the Americas should have been inclusive.”

President Alberto Fernández of Argentina also lashed out at the United States and called for a change in the rules that allowed Mr. Biden, as the host of the summit, to decide who was invited to the gathering.

“We definitely would have wished for a different Summit of the Americas,” he said. “The silence of those who are absent is calling to us.”

Los Angeles, the nation’s second-largest city, was transformed into a major geopolitical stage this week as Mr. Biden convened the summit, which takes place roughly every three years and brings together representatives from countries in the Western Hemisphere.

Mr. Biden said that the United States and Latin American countries will announce on Friday a joint “Los Angeles Declaration on Migration and Protection,” describing it as “a transformative new approach to invest in the region and solutions that embrace stability.” The declaration is expected to include Spain and Canada, in addition to the Latin American countries.

It will contain four pillars: stabilization and assistance to countries hosting migrants; new legal pathways for foreign workers; a joint approach to border protection, including tackling smuggling networks; and a coordinated response to historic flows across the U.S.-Mexico border.

But leaders from Colombia and Ecuador, which have recently announced separate programs to provide temporary legal status to up to three million Venezuelan migrants, said they needed more American investment and better trade terms to help their economies absorb the newcomers.

Marta Lucía Ramírez, the Colombian vice president and foreign minister, told The New York Times that the Biden administration should do more to provide loans to Colombian businesses. She said migration to the United States “can be stopped through investment that generates employment in each country.”

The Biden administration has announced $1.9 billion in pledges by companies to invest in Latin America. But President Iván Duque of Colombia said that less than 30 percent of the money that the international community pledged last year to help his government integrate Venezuelan migrants has been delivered.

“We need to match pledges with disbursements,” Mr. Duque said.

About six million displaced Venezuelans have fled the economic and political turmoil of their home country in the past five years, to Colombia, Peru and Ecuador, among other countries. Central Americans facing gang violence and climate change have sought fresh starts in Mexico as well as the United States. Hundreds of thousands of Nicaraguans targeted by a crackdown on dissent have moved to Costa Rica, where about 10 percent of the population consists of refugees.

U.S. Border Patrol agents encounter some 7,000 to 8,000 people each day after crossing the southern border into the United States. They include record numbers of migrants from Cuba, where economic hardship has caused food shortages. Haitians fleeing lawlessness and lack of opportunity in their home country have also been arriving by land and sea.

Leading up to the summit, the Biden administration scrambled to avoid the embarrassment of a boycott by key leaders — only to find its overtures rejected.

American officials spent weeks negotiating with the Mexican government, trying to find a way to entice President Andrés Manuel López Obrador to the meeting in Los Angeles. Vice President Kamala Harris called the leader of Honduras to persuade her to come. Top aides were dispatched to try to sway the leaders of El Salvador and Guatemala.

Nothing worked. The heads of state in all four countries have refused to attend the meeting, a blow to Mr. Biden at a moment when he sought to project unity and common purpose across the Western Hemisphere.

The Salvadoran president, Nayib Bukele, would not even get on the phone with Secretary of State Antony J. Blinken, according to four people familiar with the outreach who were not authorized to speak publicly.

The absences have cast doubt on the relevance of a summit that was meant to demonstrate cooperation among neighbors but has instead loudly broadcast rifts in a region that is increasingly willing to defy American leadership.

“It shows the deep divisions in the continent,” said Martha Bárcena, the former Mexican ambassador to the United States. The leaders who decided against attending, Ms. Bárcena said, are “challenging U.S. influence, because U.S. influence has been diminishing in the continent.”

As he closed the first working session on Thursday, Mr. Biden acknowledged the rift.

“Notwithstanding some of the disagreements relating to participation, on the substantive matters, what I heard was almost uniformity,” he said, almost pleading with his counterparts from other countries.

Natalie Kitroeff and Maria Abi-Habib and Soumya Karlamangla contributed reporting.