A senior American diplomat who oversees Haiti policy has resigned, two U.S. officials said, submitting a letter to the State Department that excoriated the Biden administration’s “inhumane, counterproductive decision” to send Haitian migrants back to a country that has been wracked this summer by a deadly earthquake and political turmoil.

The diplomat, Daniel Foote, was appointed special envoy to Haiti in July, just weeks after President Jovenel Moïse was shot in his bedroom during a nighttime raid on his residence. Mr. Foote, a former ambassador to Zambia and acting assistant secretary for international narcotics and law enforcement affairs, did not respond to messages for comment on Thursday morning.

In his stinging resignation letter, dated Wednesday, Mr. Foote criticized the Biden administration for deporting some of the thousands of the Haitian migrants who had traveled to the Texas border from Mexico and Central America in recent days.

“I will not be associated with the United States’ inhumane, counterproductive decision to deport thousands of Haitian refugees and illegal immigrants to Haiti, a country where American officials are confined to secure compounds because of the danger posed by armed gangs in control of daily life,” Mr. Foote wrote in the letter, which was first reported by PBS NewsHour. Its authenticity was confirmed by a senior State Department official and a congressional official.

Mr. Foote also blasted a “cycle of international political interventions in Haiti” that “has consistently produced catastrophic results,” and he warned that the number of migrants to American borders “will only grow as we add to Haiti’s unacceptable misery.”

In May, the Biden administration extended temporary protected status for 150,000 Haitians already living in the United States. But tens of thousands more Haitians have attempted to cross into the United States since then despite not qualifying for the program.

Mr. Foote was said to have pushed for greater oversight and responsibilities in his job as envoy to Haiti, efforts that were rejected by senior State Department officials.

In his resignation letter, Mr. Foote confirmed that “my recommendations have been ignored and dismissed.”

“Our policy approach to Haiti remains deeply flawed,” he wrote.

The rise in Haitian migration began in the months after President Biden took office and quickly began reversing former President Donald J. Trump’s strictest immigration policies, which was interpreted by many as a sign that the United States would be more welcoming to migrants.

The U.S. Border Patrol said that more than 9,000 migrants, mostly from Haiti, were being held in a temporary staging area under the Del Rio International Bridge in Texas as agents worked as quickly as they could to process them.

This week, about 300 Haitians were deported back to Haiti — the first among some 14,000 migrants who authorities in the country expect to be returned over the next month. Haitian officials have pleaded with the United States to grant a “humanitarian moratorium,” amid widespread instability.

But the Biden administration, facing the highest level of border crossings in decades, has enforced policies intended to slow the entry of migrants. On Monday, Alejandro N. Mayorkas, the secretary of the Department of Homeland Security, said newly-arrived Haitians would not be covered by a temporary residence order that protects those who had entered the United States before July 29.

“We are very concerned that Haitians who are taking this irregular migration path are receiving false information that the border is open or that temporary protected status is available,” Mr. Mayorkas said during a news conference on Monday in Del Rio, Texas. “I want to make sure that it is known that this is not the way to come to the United States.”

Officials at Haiti’s Embassy in Washington did not respond to messages for comment Thursday morning.