The Biden administration has warned the European Union and several member states to crack down on travelers from some Central Asian nations, according to people familiar with the request, part of a broader effort to prevent possible terrorists from slipping into the US.

The requests follow an incident earlier this year in which a group of migrants entered the US from the southwest border with the aid of a smuggler with ties to Islamic State. That raised alarms about passengers transiting through airports in Europe who may eventually try to gain illegal entry into the United States.

US officials have requested that the EU require airport transit visas for nationals from Uzbekistan and Tajikistan, according to two of the people familiar with the matter who spoke on condition of anonymity to discuss it.

White House spokespeople declined to comment.

CNN reported in August that the FBI had been investigating more than a dozen migrants from Uzbekistan and other countries who sought asylum at the southern border. The authorities realized the connection with an ISIS-linked smuggler only after the migrants were in the US.

An urgent classified intelligence report was circulated to President Joe Biden’s top cabinet officials, CNN reported. The FBI did not identify any specific terrorism plot, the agency said in a statement at the time.

Terrorism is dominating headlines after Hamas militants entered Israel and killed more than 1,300 people and kidnapped about 200 others in an unprecedented assault. The US request for transit visas was made before the Hamas attack, according to people familiar with the matter.

Migration has become an increasingly divisive issue as American cities, many led by Democratic mayors, struggle to provide resources for the migrants. Republicans ahead of next year’s presidential election have assailed Biden as lax on border security.

The Biden administration earlier this month announced plans to add roughly 17 miles of barriers along the Rio Grande in Texas, one of the busiest spots for migrants coming from Mexico. Biden, though, said he did not believe a border wall would work and had unsuccessfully sought to redirect the funds.

One person familiar with the demarches — a diplomatic term for formal warnings — that the Biden administration sent to the EU and some member states described them as part of an ongoing effort to learn about individuals who may attempt to enter the US, and to prevent them from traveling irregularly to the southwest border in the first place.

The EU has told the US the best way to deal with the issue would be directly with member states because introducing new rules across Europe would take longer, one of the people said.

The issue may come up when Biden meets the EU’s Ursula von der Leyen and Charles Michel on Friday, the person said.

Thousands of Uzbeks are selling their assets to be able to pay for tickets and middlemen to reach the United States. This is prompted by the devaluation of the Turkish lira and the Russian ruble, the main destinations of Uzbek workers. Growing corruptio and inequality in Uzbekistan is exacerbating the situation.