(CNN) Russian spies are ramping up their intelligence-gathering efforts in the US, according to current and former US intelligence officials who say they have noticed an increase since the election.

The officials say they believe one of the biggest US adversaries feels emboldened by the lack of a significant retaliatory response from both the Trump and Obama administrations.

"Russians have maintained an aggressive collection posture in the US, and their success in election meddling has not deterred them," said a former senior intelligence official familiar with Trump administration efforts.

Russians could also be seeking more information on Trump's administration, which is new and still unpredictable to Moscow, according to Steve Hall, retired CIA chief of operations.

"Whenever there is a deterioration of relations between countries — the espionage and intelligence collection part becomes that much more important as they try to determine the plans and intentions of the adversarial government," Hall said.

Since the November election, US intelligence and law enforcement agencies have detected an increase in suspected Russian intelligence officers entering the US under the guise of other business, according to multiple current and former senior US intelligence officials. The Russians are believed to now have nearly 150 suspected intelligence operatives in the US, these sources said. Officials who spoke to CNN say the Russians are replenishing their ranks after the US in December expelled 35 Russian diplomats suspected of spying in retaliation for election-meddling.

"The concerning point with Russia is the volume of people that are coming to the US. They have a lot more intelligence officers in the US" compared to what they have in other countries, one of the former intelligence officials says.

The FBI, which is responsible for counterintelligence efforts in the US, would not comment for the story.

Fueling law enforcement officials' concern is that the Russians are targeting people in the US who can provide access to classified information, in addition to ongoing efforts to hack the US government for intelligence, according to several of the officials. In some cases, Russian spies have tried to gain employment at places with sensitive information as part of their intelligence-gathering efforts, the sources say.

But that hasn't stopped the State Department from issuing the temporary duty visas — also known as TDY — to the suspected Russian intelligence officers. US intelligence officials who spoke to CNN expressed concerns about the number of temporary visas the State Department has issued to Russian travelers. The issue is not a new one between State and intelligence but has continued even after the intelligence findings of Russia meddling in the US election.

A State Department official would not comment specifically on the visas that have been issued, citing confidentiality under the Immigration and Nationality Act, but said "the United States is open to working with Russia where we can find areas of practical cooperation that will benefit the American people." The official adds, "Where we do not see eye to eye with Russia, the United States will continue to stand up for the interests and values of America, our allies and our partners."

DHS would not comment on the Russia visas specifically but said there is an extensive process for granting visas.

"The visa process involves multiple security checks, including screening of applicants against a wide array of criminal and terrorist databases to verify the individual's identity and to detect derogatory information that might lead to an inadmissibility determination, as well as an in-person interview with the applicant," according to a statement explaining the process.

Former State Department spokesperson Rear Admiral John Kirby says it's a complicated issue.

"To deny a visa, there has to be concrete reasons to do it," said Kirby, a CNN military and diplomatic analyst. "Sometimes they bring people over on legitimate business only, that's true. But sometimes the spies they send over here come wrapped inside the veneer of legitimate business. They blur those lines pretty well. And that's one way they try to get around the visa issue."

In some cases, the FBI uses surveillance to track the suspected Russian intelligence officers as part of a counterintelligence effort. That's how the US was able to identify and expel the 35 Russian diplomats last December, officials explained. US law enforcement officials say some of the Russian diplomats have violated protocol by leaving the Washington, DC, area without notifying the State Department. Russia has similar rules in place for US diplomats in Russia.

The issue was alluded to in a recent exchange between Republican Sen. Tom Cotton of Arkansas and Bill Priestap, the FBI's Assistant Director of the Counterintelligence Division, during a hearing about Russia on Capitol Hill. "Does it complicate you and your agent's efforts to conduct your counterintelligence mission, to have Russian nationals wandering around the country more than 25 miles outside their duty assignment?" Cotton asked. Priestap responded, "Sure. If that were to happen, that would absolutely complicate our efforts."

One flashpoint in US-Russia relations: The US shut down Russian diplomatic compounds in December that US officials believe were outfitted with sophisticated surveillance equipment targeting US military and civilian infrastructure. Russian officials have pressed the US to return the facilities in a bid to improve relations. One former administration official said that the US even watched as Russians removed suspected surveillance equipment from the compounds when they were evicted. Russia has denied the compounds were being used for intelligence gathering.

A spokesperson for the Russian embassy in Washington did not respond to requests for comment.

Even after the meddling in the US elections in 2016, the US has been slow to take measures to respond to the intelligence threat, current and former US officials say.

Partisan political disagreements over the Russian activity -- and President Donald Trump's reluctance to accept intelligence conclusions about Russia's meddling in the election -- has slowed efforts to counter the threat, current and former officials say.

US intelligence is also uneasy about ongoing Russian efforts to infiltrate US infrastructure. At a May Senate hearing on national security threats, top intelligence officials expressed concerns that the widespread use of cyber-security software in the US made by Kaspersky Labs based in Moscow could be used as a tool to accomplish that. The Russian-based company's anti-virus products have become popular in the US consumer market.

But now US government agencies are concerned that Russian security services may be able to use the software for espionage or to help access otherwise secure networks, according to US officials briefed on the matter. Kaspersky products are also commonly used in equipment bought by US government agencies. The top officials from the FBI, CIA, Defense Intelligence Agency and the National Security Agency all testified at the May hearing that they wouldn't allow Kaspersky software in their computer networks. But US government contractors may still use the products. The officials would not detail their concerns in an open hearing, citing the classified nature of the information.

Kaspersky has denied any ties to the Russian government and says it has never helped, and wouldn't assist, with any government's espionage efforts. In a statement to CNN, Kaspersky Lab says, "As a private company, Kaspersky Lab has no ties to any government, and the company has never helped, nor will help, any government in the world with its cyberespionage efforts."