A US embassy statement said it was "deeply concerned by the circumstances surrounding the cancellation of a journalism training session in Colombo over the weekend".
The training was focussed on digital security for Jaffna-based journalists and financially supported by the US government.
The statement said the cancellation of the event "was accompanied by a series of threats and intimidation of participants and other journalists around the country".
This included threats against at least one journalist who made a statement criticizing the government over the cancellation.
"This marks the third training session to be cancelled in Sri Lanka since May, including others that were disrupted by well-organised protestors threatening harm to the training participants.
"Even when present, the police have not provided security for threatened journalists or taken action against the groups making the threats," it said.
The embassy said it was also "gravely concerned" about threats by security forces against journalists covering the July 25 Jaffna court proceedings related to an 11-year-old victim of sexual abuse.
The alleged criminals in this case were the Sri Lankan Navy personnel from Karainagar in Jaffna.
"These events continue a troubling pattern of impunity for those who interfere with both freedom of speech and freedom of assembly in Sri Lanka.
"We have raised our concerns with the government of Sri Lanka and the Sri Lankan embassy in Washington, and strongly urged the government to take immediate steps to ensure the protection of fundamental rights for journalists and for all Sri Lankans."