CARACAS, Venezuela — The "biological" threat was gathering on the western border, Venezuela's socialist government claimed. So, besieged President Nicolás Maduro, ever vigilant against potential invasion, dispatched gun-toting reinforcements to the frontier.

The 57-year-old authoritarian wasn’t worried about the Colombian army. Rather, he was targeting his own people — Venezuelan migrants abroad, left jobless by the coronavirus pandemic, now returning home.

Many are stealing in from Colombia, entering the country through illegal crossings, without testing for the novel coronavirus. Maduro says they’re fueling a dangerous spike in cases in this uniquely vulnerable nation.

“Those who cross [back home] illegally, you are killing your families,” Maduro said in a televised speech last week. “The Colombian virus has sneaked everywhere, and is killing good people.”

Venezuela’s U.S.-backed opposition says Maduro’s government is making a bad situation worse by penning new arrivals in ill-equipped quarantine centers, where opposition figures say the virus is spreading.

The sides agree on at least one point. This South American nation, whose tattered health system, experts say, is among the least prepared in the world to cope with the pandemic, is now belatedly witnessing a long-feared coronavirus outbreak, with cases soaring and hospitals overwhelmed.

More than 5 million people have fled starvation, poverty and government repression in Venezuela in recent years, with most settling elsewhere in Latin America, where the poorest have scraped out precarious lives as domestic workers, laborers and street vendors.

But as the coronavirus has torn through the region, and countries from Mexico to Argentina have locked down, many of these refugees, left jobless and hungry on the streets of Quito, Lima and Bogotá, are heading home. In recent weeks, a rush of 60,000 returnees has caused bottlenecks at Colombian border-control points. Some have spilled over to illegal crossings, known as trochas, along the lawless frontier between Colombia and Venezuela.

Waiting for the returnees are Maduro’s soldiers and mandatory quarantines in the border states of Zulia, Táchira and Apure, the region seeing by far the most alarming spike in cases. Returnees say they are being held without face masks or social distancing.

Mayra Jiménez, a 37-year-old manicurist, lost her job in Colombia when the country locked down. At the border, she said, she was given two rapid tests — one by the Colombians, the other by the Venezuelans. Both, she said, came out negative.

Five days after returning home, however, she tested positive. She was sent for 35 days to a public hospital in Apure state. The facility had no working X-ray machine, so at one point she was transferred to the nearest hospital that had one — seven hours away.

“I remember asking God to take care of my family,” said Jiménez, who has since recovered. “I swore to God that I would never leave my country again if He saved me.”

Border-state hospitals, already ravaged by years of equipment breakdowns and chronic shortages of medicines and supplies, are struggling to cope with rising demand.

Venezuela has reported more than 10,000 coronavirus cases and 96 deaths. But with limited testing and the authoritarian government in control of the data, both are widely believed to be undercounts.

Staff at University Hospital Maracaibo last week said the facility, designated by the government as the city’s primary hospital for the coronavirus, was averaging 500 patients with symptoms, double the number a month ago. All eight ICU beds and 10 ventilators were in use, with dangerously long waiting lists for others who needed them.

One medical resident spoke of alarming mortality.

“The number of deceased is increasing,” she said. “We’re seeing normal days in which we lose up to 10 patients.”

Staff say most of Maracaibo’s dead have not yet been included in the official count because tests are sent to Caracas for processing and take up to a month to get back.

The resident, who, like other staff, spoke on the condition of anonymity for fear of government reprisal, said conditions at the hospital were so bad that many infected people are choosing to forgo care.

“There’s no air conditioning, and we have to endure 100-degree heat,” she said. “It’s suffocating.”

There’s also no running water, so nurses and doctors wash their hands in buckets. The hospital is one of the few in Zulia state to have recently received face masks, but they’re being reserved mostly for health workers. The lack of personal protective equipment for janitors has meant that ward floors sometimes go days without cleaning.

The local physicians’ association says 44 doctors and nurses in Zulia have been stricken by the coronavirus. Nine have died, according to the nonprofit Doctors United for Venezuela. A dozen remain in intensive care.

Staff say the pro-government hospital administration has threatened them with criminal charges if they do not keep reporting for duty. The sicknesses and deaths of doctors and nurses, they say, have worsened already severe staff shortages.

One nurse who tested positive for the coronavirus was escorted to a quarantine center by armed soldiers.

“Now there’s practically no staff,” she said.

Venezuela was among the first nations in South America to impose a strict national lockdown. That move, along with the country’s relative isolation — even before the pandemic, only a handful of airlines flew here — appeared initially to keep infections low.

But cases have soared ninefold since late May, according to the official count, with the highest spikes in states along the Colombian and Brazilian borders.

The data suggests that 70 percent of new infections are from community transmission. But doctors say the high concentration of cases near the Colombian frontier suggests a link with border crossings. Maduro’s government is openly blaming returnees, and the traffickers helping them, who are avoiding legal crossings and the tests that are administered there.

“Any person who violates the immigration system and enters the country [illegally] will be considered a biological weapon,” said Lisandro Cabello, a senior member of Maduro’s party in Zulia. On Tuesday, Maduro announced a Gmail address for Venezuelans to denounce countrymen who enter at illegal crossings.

Maduro has accused the Colombians of lax procedures and weak border control. The Colombians reject the charge.

“What’s really sad is that over 5 million Venezuelans left the country, and now they’re trying to stigmatize the few thousands who are trying to return to their own land,” said Felipe Muñoz, a senior adviser to Colombian President Iván Duque.

Some doctors who are allied with the Venezuelan opposition say the government’s quarantine facilities, which lack running water, electricity, soap or masks, are promoting the spread of the coronavirus.

“It’s the migrants,” said Julio Castro, the epidemiologist who is spearheading a team monitoring the crisis for the opposition. “We’re sure about that. But our theory is that they’re getting sick in the camps, not that they’re all somehow entering the country as coronavirus carriers.”

Aid workers and doctors express concern about conditions in the quarantine centers but say authorities are testing rigorously. They also say some returnees are bribing soldiers to avoid being quarantined.

“People in the border can pay $20 to the guards to get them in,” said Martin Carballo, an infectious-disease doctor in Caracas. “Now we have a well-established community transmission.”

Maduro’s government and the Venezuelan opposition signed an agreement last month to coordinate relief efforts through the Pan American Health Organization. The deal would use government funds frozen by the U.S. and European governments to purchase protective gear, medicines and medical equipment.

Jarbas Barbosa da Silva, the health organization’s assistant director, said he hoped for a final deal, with transparency measures required of the Venezuelan government in exchange for the supplies, this month. It can’t come soon enough.

The organization has already supplied Venezuela with shipments of personal protective equipment.

“But of course they need more,” Barbosa said. “We are still trying to get more. When health workers are not protected, it’s a problem. Other health workers will be afraid to go to the hospitals.

“It’s very important the process move more quickly.”

Faiola reported from Miami.