The offer to employees at the state-owned oil giant was compelling: Be among the first in China to take a coronavirus vaccine.

The employees at PetroChina could use one of two vaccines “for emergency use” to protect themselves when working overseas as part of China’s ambitious infrastructure program, according to a copy of the notice, which was reviewed by The New York Times. They would effectively be guinea pigs for testing the unproven vaccines outside the official clinical trials.

The offer was backed by the government. It stressed that data from clinical trials showed that the products, both made by Sinopharm, were safe. It did not mention the possible side effects, or warn against the false sense of security from taking a vaccine that regulators hadn’t approved.

“I don’t think this is right ethically,” said Joan Shen, the Shanghai-based chief executive officer of the pharmaceutical firm I-Mab Biopharma.

The unorthodox move, to test people separately from the normal regulatory approval process, reflects the formidable challenge facing China as it races to develop the world’s first coronavirus vaccine.

Eager to find a long-term solution to the outbreak and burnish their scientific credentials, Chinese companies are rushing to get as much data as possible on their vaccines to prove they are safe and effective. In China, they are selectively testing their vaccines on small pools of people like the PetroChina employees — an approach that does not count toward the regulatory process but could bolster their own confidence in the vaccines. In Brazil and other countries, they are conducting clinical trials, going through the normal regulatory channels.

The dual strategy, though, is risking scientific setbacks and political backlash, potentially undercutting China’s efforts.

Such “emergency use” is rare, and the taking of unapproved vaccines is typically reserved for health care professionals. Although the government has stressed that taking the vaccine is voluntary, the state-owned workers and soldiers could feel pressure to participate.

As Chinese companies also look beyond their borders to test the vaccine, they are running into mistrust and skepticism. Health experts have questioned why the Canadian government is allowing CanSino Biologics, which has teamed up with the People’s Liberation Army, to run human trials in the country. Rumors have spread about the authenticity of a Chinese-made vaccine that is being tested in Brazil, as supporters of President Jair Bolsonaro cast doubt.

The strategy is born out of necessity.

Chinese companies cannot find enough candidates at home to conclusively determine whether their vaccines would prevent infections, a problem faced by research institutes and pharmaceutical makers in countries that have largely tamed the coronavirus. Phase 3 trials, the final stage before approval, require vaccines to be tested in roughly tens of thousands of volunteers in places with large, active outbreaks.

Along with the testing at the oil company, Sinopharm, which has completed Phase 2 trials for two products, has injected the vaccine into its chairman and other senior officials, according to the State-owned Assets Supervision and Administration Commission, or SASAC, the government agency managing all employees at state-backed companies. The Chinese government has allowed the CanSino-military vaccine to be given to its armed forces, a first for the military of any country.

Yang Zhanqiu, a virologist at Wuhan University, was skeptical about the decision to give vaccines to state-owned employees for business travel.

“It does not make much sense at all because the length of time that the employees take to travel is not the same, the locations may be different, and it is not easy to do tracking and monitoring,” Dr. Yang said. “It may just be a psychological comfort for employees.”

A PetroChina employee based overseas confirmed that his colleagues in China had been invited to take the vaccines. PetroChina, SASAC and Sinopharm did not respond to requests for comment.

Such testing does not help the companies clear any regulatory hurdles, since it is not part of the official clinical trials. Mainly, Chinese companies could use it to give themselves extra reassurance that the vaccines are safe, presuming they do not discover any problems.

“If you are a regulatory body, if you play by the rules, if you are hard-nosed about it, you say this is very wrong,” said Ray Yip, the former head of the Gates Foundation in China.

Dr. Yip added that it would be useful for company executives to know that they had given the dose to “a couple of thousand people, but no one has dropped dead, so that’s pretty good.”

Dr. Yip said the people taking the vaccines should read up on reports of the safety data and make an informed decision. He said he would be willing to take it.

“If you offer that to me saying it’s safe and there’s an 85 percent chance that it works, would I take it today?” he said. “You know what, I probably will. Because then I don’t have to worry.”

In a post on its official WeChat account, a government agency reported that the “vaccine pretest” on Sinopharm employees showed that antibody levels were high enough in subjects to combat the coronavirus, indicating that it was safe and effective. The agency did not make clear what vaccine the employees had taken. The Paper, a newspaper owned by the Shanghai government, separately said 180 employees had taken the vaccine.

Last month, the Beijing Institute of Biological Products, which is developing one of Sinopharm’s vaccines, published its preclinical data in a peer-reviewed journal, Cell, saying the vaccine induced high levels of antibodies in macaques and protected against Sars-CoV-2, the virus that causes Covid-19.

The Wuhan Institute, which Sinopharm owns, said its vaccine had caused no adverse reactions among volunteers, according to Xinhua, China’s official news agency. Volunteers achieved full antibodies after two doses in a 28-day program.

While the early results in those small groups are promising, Chinese companies must strike deals in other countries to ultimately pass regulatory muster in China and the rest the world.

In June, Sinopharm began the third phase of clinical trials in Beijing, Wuhan and Abu Dhabi, becoming the first company to enter the final regulatory stage. China’s Sinovac Biotech is teaming up with Instituto Butantan in Brazil, which has the world’s second-highest case count after the United States.

The process has been politically fraught in Brazil, where Mr. Bolsonaro has downplayed the threat, even as he later contracted Covid-19. His son blamed China for the pandemic.

Shortly after the vaccine deal, a fake meme started spreading that said the vaccine had been tested only on monkeys and never on humans. “If this vaccine is so promising, then why not test it in China, where this damned virus appeared, instead of testing it on the citizens of São Paulo?” it said.

Dimas Tadeu Covas, the director of Butantan, said that he was impressed with Sinovac’s preliminary results and that the vaccine “has the greatest potential for success.” He cited results from Sinovac’s Phase 1 trials that showed no adverse effects and Phase 2 trials that showed 90 percent protection against Sars-Cov2.

“I know vaccines, and I am betting a lot on this one,” Dr. Covas said.

Despite the political backlash, about 600,000 people signed up for the trials just 24 hours after the recruiting process went live this week.

João Doria, the governor of São Paulo, the state where testing is being conducted, said his priority was to “find tools which can help save lives.” “In the middle of a pandemic, you can’t prioritize ideology or political factors over life,” he said in an interview.

Ralcyon Teixeira, an infectious-disease specialist and director of the medical division at the hospital Emílio Ribas in São Paulo, said he was worried that the “politicization” of the Sinovac vaccine could hinder the introduction of what he believed could be an effective treatment.

“It’s been four months of just Covid,” he said. “We are tired of seeing so many deaths, so many bad and tragic situations, so I think that we are seeking out hope that this vaccine will work.”

Amber Wang and Liu Yi contributed research.