Thousands of Venezuelans in limbo on the Mexican side of the U.S. border are becoming increasingly anxious as their numbers swell and they are crammed into makeshift camps to wait for a change in U.S. policy that is not likely to happen anytime soon. 

The tensions boiled over at El Paso Tuesday, when dozens of migrants — one of them waving a giant Venezuelan flag — attempted to cross the Rio Grande into the United States and had to be repelled by Border Patrol agents firing pepper balls.

Video posted on social media from the scene showed hundreds of migrants amassed along the southern banks of the river waiting to try to cross the slow-moving and shallow water into downtown El Paso. The dozens who actually did cross reversed course when confronted by a phalanx of Border Patrol agents.

In a statement Wednesday, the commissioner of the U.S. Customs and Border Protection agency, Chris Magnus, said the pepper balls — “a less-than-lethal system,” as he called them — were deployed after several of the migrants became combative and “physically assaultive.”

“In accordance with the need to ensure any use of force by CBP personnel complies with our policies, CBP’s Office of Professional Responsibility is reviewing this incident,” he said.

A report in the El Paso Times described the scene Tuesday as “tense and chaotic,” with one migrant assaulting an officer with a flagpole and others hurling rocks at the agents. Three shots were fired by officers, according to the report.

On Monday, some of the Venezuelans marched through downtown Ciudad Juárez, El Paso’s sister city across the border, to demand that the Biden administration rescind an edict issued October 12 that requires Venezuelans who are caught crossing the border illegally to be deported to Mexico.

Previously, the migrants were released and allowed to await decisions on their requests for asylum in the United States. Nearly 200,000 Venezuelans were processed and released at the border in the fiscal year that ended September 30 — 34,000 in September alone.

President Biden issued the new rules after the long-simmering border crisis burst into the headlines when Republican governors began bussing asylum-seeking migrants from tiny border towns to large urban centers run by Democrats in the northeast. Recent polls have suggested that immigration is among the top concerns of voters in this month’s midterm elections.

Mr. Biden’s deal with the Mexican president, Andrés Manuel López Obrador, allows the administration to return an undetermined number of Venezuelan migrants to Mexico if they are caught by the Border Patrol — a policy first implemented by President Trump in early 2019. In exchange, Mr. Biden promised 65,000 additional work visas for Mexican, Central American, and Haitian migrants, and agreed to allow up to 24,000 Venezuelans into the country — but only by air, not at the border.

On Friday, U.S. and Mexican officials said 7,500 applications for the new airborne visas were being processed and the first 100 Venezuelans had been approved to fly. Biden administration officials reported that about 150 Venezuelans were crossing the border from Mexico daily, down from more than 1,200 before the policy was announced.

Migrant aid workers in Juárez say Venezuelans are still arriving in the city by the thousands and are, increasingly, pitching tents near the river in hopes that the new policy will be reversed. As many as 2,000 have been deported to Mexico since the policy was enacted, they say.

The Mexican government had been telling those expelled from America that they had 15 days to leave the country, according to the El Paso Times, but has since started giving them 180-day visas to remain.