WASHINGTON—The U.S. says it won't shy away from raising its serious concerns over human rights in Cambodia ahead of a scheduled visit by President Barack Obama.

Obama is expected to attend a summit of East Asian leaders in Phnom Penh (puh-NAHM' pehn) Nov. 18-20.

In a commentary published this week in The New York Times, exiled Cambodian opposition leader Sam Rainsy urged Obama to stay away to avoid conferring international legitimacy on Prime Minister Hun Sen, who has ruled Cambodia virtually unchallenged since a 1997 coup.

Kurt Campbell, the top U.S. diplomat for East Asia, said Thursday that the U.S. was concerned about steps Cambodia has taken against opposition groups, civic organizations and individuals.

Rights groups have accused the U.S. of failing to press Cambodia on rights abuses.