As Belarus’s campaign of weaponized migration spreads across the Baltics and into Poland, the European Union must treat it as an attempt to destabilize the entire bloc and react swiftly, even if that means helping to construct a fence along its external border, Lithuanian President Gitanas Nauseda said in an interview with The Washington Post.

Belarusian strongman Alexander Lukashenko is increasingly desperate, Nauseda said, warning that as new sanctions pile up against his regime, Lukashenko will only intensify his “hybrid attack on the democratic world.”

According to the account of the Lithuanian government, this offensive involves facilitating the travel of thousands of mostly Iraqi migrants to Minsk, where they are then ushered over the border and into one of three E.U. member states that neighbor Belarus. The number of undocumented migrants entering Lithuania rose sharply for weeks, until sustained diplomatic efforts and an aggressive border patrol strategy led to a recent decline. But now, Latvia and Poland are reporting significant increases.

“The Belarusian regime is a desperate regime, trying to survive at any cost,” Nauseda said. “We have to stay united. . . . This is our advantage compared to dictatorships. Democratic countries are stronger if they are united.”

The European Union this week made its most significant commitment yet to helping Lithuania secure its border, approving more than $40 million in emergency funding, which will help pay for migrant detention facilities, asylum processing, food and medical care.

Lithuanian officials welcomed the money, but many leaders — including Nauseda — want the European Union to help pay for the construction of a fence along the 420-mile border between the two former Soviet countries.

After visiting Lithuania, the top E.U. migration official endorsed the idea of a fence, but so far the bloc has refused to pay for it.

“We’ve been very clear about the fact that the commission does not finance fences or barriers,” E.U. spokesman Adalbert Jahnz said at a Tuesday news conference.

But Nauseda said his country will “talk and negotiate further” with the other members of the bloc, and he’s confident the European Union will provide some support, such as supplying equipment or technical assistance.

“I expect there will be no problems for us because the European Union understands very well this is not only a hybrid attack against Lithuania,” Nauseda said, “but this is also an attack against the European Union as a whole.”

Since Lukashenko claimed victory in last summer’s presidential election, which the opposition and Western leaders rejected as rigged, he has only escalated his crackdown at home and grown more defiant abroad, experts have said. The Olympics brought greater international attention to his regime’s tactics, when Belarusian runner Krystsina Tsimanouskaya accepted a humanitarian visa from Poland, saying she was afraid to return home after criticizing her country’s team.

On Monday, the first anniversary of the contested election, the United States, Britain and Canada imposed another round of sanctions, targeting Belarus’s potash and petroleum sectors, along with prominent companies and business leaders. President Biden accused Lukashenko of carrying out a “brutal campaign of repression to stifle dissent.”

That day, the Belarusian president held an eight-hour news conference, criticizing the West and denying that he has repressed civil society in his country or used migrants to retaliate against the European Union for its sanctions.

“We are not blackmailing anyone with illegal immigration,” he said. “We’re not threatening anyone. But you have put us in such circumstances that we are forced to react.”

Lithuanian officials say they have documents and other intelligence to prove the involvement of the Belarusian government and state-owned businesses in migrant trafficking. Last week, Lithuania’s Interior Ministry published helicopter footage it said showed Belarusian border guards escorting groups of migrants to the border.

What was for weeks a steady stream of migrants crossing into Lithuania has recently slowed. In all, authorities have detained and set up makeshift camps for more than 4,100 people, who mostly arrived in July. In early August, Lithuania began the controversial strategy of turning migrants back to Belarus, which prevented another 1,300 people from entering, Nauseda said.

And after weeks of E.U. and Lithuanian lobbying, the Iraqi government suspended all flights to Belarus, shutting down the primary route Lukashenko used to bring migrants into the country, officials say. Just 271 people entered Lithuania during the first week of August, compared with about four times that the week before.

But as numbers fell in Lithuania, they rose in neighboring Latvia and Poland. Nearly 500 people crossed into Poland from Belarus during the first week of August, nine times the number from the week before. Another 349 arrived over the weekend, and Poland and Lithuania issued a joint statement denouncing “the criminal actions of the Lukashenko regime.”

In Latvia, the government ordered a state of emergency after crossings from Belarus increased from 22 to 107 week over week. On Monday, another 103 people entered. Latvia’s declaration effectively shuts down its border with Belarus, allowing the military and police officers to patrol and turn migrants away without accepting asylum applications.

The numbers are small compared with the international emergency of 2015 and 2016, when 1.4 million migrants arrived in Europe. But Lithuania, with a population of less than 3 million, has not previously been a migration destination and says it is ill-equipped to handle the influx.

“It is the first crisis of this sort,” said Dovile Jakniunaite, head of the international relations department at Vilnius University. “I think everyone sees the government was not prepared. No government would have been prepared. It was pretty obvious that they improvised on the spot.”

Human rights activists have criticized the government for pushing through a new law that fast-tracks the asylum process and allows authorities to detain migrants for as long as six months without a court order.

Nauseda defended his country’s treatment of the migrants as humane, saying they’ve been given adequate food, water and shelter, but said caring for thousands of people at the border has been a challenge.

“This is really a relatively big number, and we have limited resources, but we try to do our best,” Nauseda said.

So what will happen to the migrants now in Lithuania? Officials say the vast majority do not appear to qualify for asylum and will eventually be deported back to their home countries. Lithuania has already begun encouraging people to leave, even offering about $350 to anyone who agrees to return to their country of origin. Nauseda said Baghdad could send empty planes to Vilnius to repatriate Iraqi citizens.

Analysts say this crisis underscores the European Union’s continued lack of a coherent migration and asylum policy, and it could resurface the same tensions that arose five years ago, as the continent struggled to deal with a surge of refugees. Lukashenko is trying to “hit them where it hurts” in a bid to sow discord, said Steven Blockmans, the director of research at the Center for European Policy Studies.

“Migration policy is really the raw nerve where Lukashenko and his regime can inflict pain on the E.U. and hope to weaken its stance overall,” he said.

The bloc is also warily eyeing the Taliban’s swift territorial gains in Afghanistan and bracing for another potential migration surge in the coming months.

On Monday, ministers from nine European governments urged the European Union to take more forceful, coordinated steps in responding to the “instrumentalization of migration.” The letter, obtained by The Post and first published by the Belgian outlet Knack, says Belarus’s tactics pose an urgent threat to the bloc.

“There is no doubt that if the European Union fails to collectively respond to this new tactic by third states,” the countries warned, “the problem will not just persist but could increase in scope and impact.”