The American government secretly expelled two Chinese embassy officials in September after they drove on to a sensitive military base in Virginia, according to people with knowledge of the episode. The expulsions appear to be the first of Chinese diplomats suspected of espionage in more than 30 years.

American officials believe at least one of the Chinese officials, who were with their wives, was an intelligence officer operating under diplomatic cover, said six people with knowledge of the expulsions. The group evaded military personnel pursuing them and stopped only after fire trucks blocked their path.

Neither Beijing nor Washington announced the September episode.

The episode in September, which neither Washington nor Beijing announced, has intensified concerns in the Trump administration that China is expanding its spying efforts in the US as the two nations are increasingly locked in a geopolitical and economic rivalry. American intelligence officials say China poses a greater espionage threat than any other country.

In recent months, Chinese officials with diplomatic passports have become bolder about showing up unannounced at research or government facilities, American officials said, with the infiltration of the military base only the most remarkable instance.

The expulsions, apparently the first since the US forced out two Chinese embassy employees with diplomatic cover in 1987, show the American government is now taking a harder line against suspected espionage by China, officials said.

Recent episodes of suspected spying add to the broader tensions between the US and China, the world’s two largest economies and biggest strategic rivals. That conflict is heightened by a trade war that President Donald Trump started in July 2018 and that shows only tentative signs of abating.

On October 16, weeks after the intrusion at the base, the state department announced sharp restrictions on the activities of Chinese diplomats, requiring them to provide notice before meeting with local or state officials and with educational and research institutions.

At the time, a senior state department official told reporters that the rule, which applied to all Chinese missions in the US and its territories, was a response to Chinese regulations imposed years ago requiring American diplomats to seek permission to travel outside their host cities or visit certain institutions.

The Chinese embassy said in October that the new rules were “in violation of the Vienna Convention”.

Two American officials said last week that those restrictions had been under consideration for a while because of growing calls in the American government for reciprocity, but episodes like the one at the base accelerated the rollout.

The base intrusion took place in late September on a sensitive installation near Norfolk, Virginia. The base includes Special Operations forces, said the people with knowledge of the incident. Several bases in the area have such units, including one with the headquarters of the navy’s elite SEAL Team Six.

The Chinese officials and their wives drove up to a checkpoint for entry to the base, said people briefed on the episode. A guard, realising they did not have permission to enter, told them to go through the gate, turn around and exit the base, which is common procedure in such situations.

But the Chinese officials instead continued on to the base, according to those familiar with the incident. After the fire trucks blocked them, the Chinese officials indicated that they did not understand the guard’s English instructions, and had simply gotten lost, according to people briefed on the matter.

American officials said they were sceptical that the intruders made an innocent error and dismissed the idea that their English was insufficient to understand the initial order to leave.

It is not clear what they were trying to do on the base, but some American officials said they believed it was to test the security at the installation, according to a person briefed on the matter. Had the Chinese officials made it onto the base without being stopped, the embassy could have dispatched a more senior intelligence officer to enter the base, the theory goes.

The Chinese foreign ministry and the Chinese embassy in Washington did not reply to requests for comment about the episode.

Two associates of Chinese embassy officials said they were told that the expelled officials were on a sightseeing tour when they accidentally drove onto the base.

The US state department, which is responsible for relations with the Chinese embassy and its diplomats, and the FBI , which oversees counterintelligence in the US, declined to comment.

Chinese embassy officials complained to state department officials about the expulsions and asked in a meeting whether the agency was retaliating for an official Chinese propaganda campaign in August against an American diplomat, Julie Eadeh.

At the time, state-run news organisations accused Eadeh, a political counsellor in Hong Kong, of being a “black hand” behind the territory’s pro-democracy protests, and personal details about her were posted online. A state department spokeswoman called China a “thuggish regime”.

So far, China has not retaliated by expelling American diplomats or intelligence officers from the embassy in Beijing, perhaps a sign that Chinese officials understand their colleagues overstepped by trying to enter the base. One person who was briefed on reactions in the Chinese embassy in Washington said he was told employees there were surprised their colleagues had tried something so brazen.