Guatemala’s Constitutional Court ruled that lawmakers in the capital, Guatemala City, needed to approve the policy before it could be carried out, and that has yet to happen. The administration has also failed to get Mexico to sign such an agreement.

Human rights advocates say it makes no sense to ask migrants to seek protection in countries like El Salvador and Guatemala, which are among the most dangerous, gang-ridden places in the world.

Tens of thousands of Salvadorans have been displaced from their homes, and the number of disappearances suggests that the official homicide rate may be considerably higher than the numbers reported by the police.

In 2018, about 46,800 Salvadorans sought asylum worldwide, ranking the country sixth in the world for new asylum seekers. In addition, according to a government study supported by the United Nations high commissioner for refugees, at least 71,500 Salvadorans have been internally displaced by violence. Overall, about 150,000 Salvadorans have become refugees or sought asylum in recent years.

“All these rules, agreements and procedural hurdles are creating a paper wall on the southern border, one that is just as inhumane, immoral, and illegal as one made of metal or bricks,” said Eric Schwartz, the president of Refugees International, an advocacy organization. “When history looks back on this period in the United States, the judgment will be harsh and unsparing.”

Since taking office in June, President Nayib Bukele of El Salvador has moved quickly to try to bring down the country’s homicide rate, sending the military into its most violent areas.

Although it had already begun to fall since 2017, when it was the highest in the world, according to the United Nations office on drugs and crime, the first three months of Mr. Bukele’s presidency showed a continued drop. According to Roberto Valencia, a Salvadoran reporter who analyzes homicide statistics released by the National Civil Police, the homicide rate in August was the lowest since 2013.