Relations between Hong Kong and the United States frayed after Beijing imposed a national security law on the city in 2020, which critics say has quelled dissent and eroded rights, and ties have further strained during the latest US-China competition.

US Consul General Gregory May said he was "disappointed" as the consulate regularly interacts with schools.

"I challenge any of you to find anything about this event... that would in any way violate the national security of any country," May told a reception marking the July 4th anniversary.

Several schools got notices from education officials to keep pupils away from consulate-hosted US Independence Day events and avoid breaching the security law, local newspaper Ming Pao reported last week.

The Education Bureau did not confirm or deny the report, only saying on Tuesday that "schools have the responsibility to play a good gate-keeper role and to enhance the sensitivity of teachers and students to national security".

A US consulate spokesperson condemned the move as "repression".

Guests at Thursday's reception included Hong Kong's commerce and economic development chief Algernon Yau and former city leader Donald Tsang.

Pro-Beijing lawmaker Regina Ip, who convenes the government's cabinet, told AFP on Wednesday that the Bureau made an "oversimplification, causing unnecessary concern".

May, who will soon become deputy head of the US embassy in Beijing, also bemoaned the treatment of pro-democracy media mogul Jimmy Lai and other "political prisoners", saying Hong Kongers have been jailed for peaceful expression of views.

Lai, 77, is in the middle of a lengthy trial for foreign collusion charges under the national security law and has been in custody since 2020.

May criticised the Hong Kong government for "transnational repression", referring to police bounties issued against 19 overseas pro-democracy activists.

"This is an attempt to restrict the free speech of people in the United States," he said, adding that it was "very disturbing" that authorities have targeted the activists' families in Hong Kong.