Washington - High-level allegations this week that India was involved in the killing of a Sikh leader on Canadian soil are renewing fears that governments worldwide are no longer afraid to cross boundaries to silence dissenting voices.

Canadian Prime Minister Justin Trudeau made the unprecedented accusation Monday, telling Canadian lawmakers that his government has 'credible allegations' of India's ties to the June slaying of exiled Sikh leader Hardeep Singh Nijjar in Vancouver.

India has rejected the charge as 'absurd.' But the episode has worried top U.S. officials who are tracking a growing number of cases in which critics are being targeted across nation-state boundaries.

'I won't speak to that particular report,' said Kenneth Wainstein, the Department of Homeland Security's undersecretary for intelligence and analysis, in response to a question from VOA about Canada's accusation against India.

'But I will say ... we have seen an increase in foreign nation state efforts to repress and persecute people they see as dissidents here in the United States,' Wainstein said during an event at a Washington think tank on Tuesday.

'We're seeing that from the PRC [People's Republic of China] police stations that are set up here and [are] being used to monitor and harass people that they think are unfriendly to the regime,' he said. 'We're seeing in [sic] other countries who are targeting people here in the United States.'

U.S. counterintelligence officials, in recent months, have repeatedly sounded the alarm about a growing number of incidents.

Most of the concern has focused on China and Iran, labeled 'significant offenders' by officials at the FBI.

The officials point to arrests, including in April, where two U.S. citizens were charged in connection with running an illegal police station in New York City's Chinatown neighborhood.

Some of Iran's efforts to silence critics have been even more brazen, including multiple plots targeting Masih Alinejad, an Iranian American human rights activist and VOA Persian TV host and a murder-for-hire plot targeting former U.S. national security adviser Ambassador John Bolton.

But China and Iran are not alone.

U.S. prosecutors point to a series of indictments against suspects linked to Belarus, Egypt and Saudi Arabia, while concerns about Russia are ever present.

According to a separate database maintained by the U.S.-based Freedom House, there were 854 physical incidents of transnational repression committed in 91 countries by 38 governments since 2014.

China ranked first in the report, followed by Turkey, Tajikistan, Russia, Egypt, Turkmenistan, Uzbekistan, Belarus, Iran and Rwanda. India also made the list.

China has consistently rejected the accusations that it engages in transnational repression, describing such allegations as a smear campaign.

Other countries have declined to address the allegations.

Still, U.S. counterintelligence officials warn the willingness of countries to cross borders to silence dissent has hit an 'inflection point,' telling reporters in April there are 'lines that are being crossed' by a growing number of governments.

U.S. officials have likewise warned that the operations to target dissidents are getting ever more sophisticated, sometimes making use of third parties who are unaware of how they are being used.

'We've seen where the private investigators are not witting to what's going on and where, in fact, the subjects are using some kind of cut-out company to mask who is behind,' an FBI official told VOA earlier this year, speaking on the condition of anonymity.

'Same thing with local police departments,' the official added.

The Department of Homeland Security's Wainstein on Tuesday said his office and others are doing everything they can to push back.

'It's a huge threat, both to those individuals, but to our sovereignty,' he said, responding to a question from VOA.

'We have people who have come to the United States, as generations have before, as a place of freedom,' he said. 'And their old regimes back in their home country are reaching out to try to intimidate them.'