The State Department said it was a direct response to "anti-democratic actions" by the Cambodian government but did not disclose which individuals would be affected. It said visa records are confidential under U.S. law.
Spokeswoman Heather Nauert called on the Cambodian government to reinstate the opposition Cambodia National Rescue Party, which was dissolved by Supreme Court order last month, and free its leader Kem Sokha, imprisoned since September. She also urged Cambodia to allow civil society and media to operate freely.
Prime Minister Hun Sen, who has held power for more than three decades, has sought to neutralize political opponents and silence critics ahead of national elections next year. Kem Sokha has been charged with trying to topple the government with U.S. support, which Washington has said is a baseless accusation.
Nauert said Cambodia's actions run counter to the Paris Peace Agreements of 1991. The United States and 18 other governments signed the accords, which ushered in democracy after the genocidal rule of the Khmer Rouge regime in the late 1970s, then occupation by Vietnam and civil war.
Secretary of State Rex Tillerson will restrict entry into the United States of "those individuals involved in undermining democracy in Cambodia," Nauert said in a statement, adding that in certain circumstances, family members of those individuals will also be subject to visa restrictions. The department cited a provision of U.S. immigration law under which individuals can be denied entry if the secretary determines it would have "adverse foreign policy consequences."
The White House has already terminated U.S. support of Cambodia's national election committee, saying last month that the July 2018 vote "will not be legitimate, free or fair."
"We will continue to monitor the situation and take additional steps as necessary, while maintaining our close and enduring ties with the people of Cambodia," Nauert said.
Cambodian government spokesman Phay Siphan said Cambodia would give the U.S. government time to understand the current situation in the country but said Washington should not only believe what the opposition is saying. Still, he said the relationship between the U.S. and Cambodia remained unchanged despite the new restrictions.
"It is the decision made by the United States government, but for Cambodia, the relationship of the two countries remains the same," he said Thursday.
Monovithya Kem, an opposition spokeswoman currently in the U.S., welcomed the visa restrictions and called for targeted financial sanctions on senior officials in Hun Sen's government. Kem, who is the daughter of Kem Sokha, urged the U.S., Japan, Australia and the European Union to coordinate responses to the "crisis" in Cambodia and help win her father's freedom.
Like many prominent opposition figures, Kem has fled Cambodia as she fears arrest.
Hun Sen has been in office since 1985 and has held a tight grip on power since ousting a co-prime minister in a bloody 1997 coup.
In recent months, the government has intensified restrictions on civil society groups and independent media outlets. In September, it shut down the English-language Cambodia Daily. Authorities have shuttered radio stations that aired programming from U.S.-funded Radio Free Asia and Voice of America, whose reports they allege are biased.
The government also expelled the U.S. National Democratic Institute, which helped train political parties and election monitors, accusing it of colluding with its opponents.
Hun Sen has moved Cambodia closer to China in recent years and become increasingly critical of Washington. However, he's been complimentary of President Donald Trump.
Speaking at Asian leaders' summit attended by Trump last month, Hun Sen praised the U.S. leader for non-interference in affairs of other nations, but complained the U.S. Embassy in Cambodia was not adhering to the policy.
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