A group of Central Americans who journeyed in a caravan to the US border to ask for asylum in a direct challenge to the Trump administration have been told by immigration officials the San Diego crossing is already at capacity.
Nearly 200 migrants, many travelling with children, had decided to apply for protection at the nation's busiest border crossing after many fled violence in their home countries, organisers said on Sunday.
The caravan got attention after President Donald Trump and members of his Cabinet called it a threat to the US.
Shortly before the migrants were expected to arrive, US Customs and Border Protection said San Diego's San Ysidro crossing would not immediately be able to handle more asylum seekers. It can hold about 300 people at a time.
"At this time, we have reached capacity at the San Ysidro port of entry for CBP officers to be able to bring additional persons travelling without appropriate entry documentation into the port of entry for processing," Commissioner Kevin McAleenan said in a statement.
"Those individuals may need to wait in Mexico as CBP officers work to process those already within our facilities."
He said the crossing could take in additional people as space and resources become available. Despite the news, about 200 migrants still started walking toward the port.
Nicole Ramos, a lawyer working on behalf of caravan members, expressed disbelief that US authorities cannot process more asylum seekers until its backlog eases.
"They have been well aware that a caravan is going to arrive at the border," she said at a news conference.
"The failure to prepare and failure to get sufficient agents and resources is not the fault of the most vulnerable among us. We can build a base in Iraq in under a week. We can't process 200 refugees. I don't believe it."
The migrants had made their way north by foot, freight train and bus over the past month, many of them saying they feared for their lives in their home countries.
Attorney General Jeff Sessions has called the caravan "a deliberate attempt to undermine our laws and overwhelm our system."
Intending migrant Wendi Yaneri Garcia said police in her hometown in Honduras jailed her for protesting the construction of a hydro-electric plant and that she received death threats after being released.
Nefi Hernandez, 24, who arrived from Honduras with his wife and baby daughter, said a gang in his hometown threatened to kill him and his family if he did not sell drugs.
