NEW DELHI — The United States has imposed an entry bar on Sri Lanka’s army chief, Lt. Gen. Shavendra Silva, citing his alleged involvement in war crimes during the final stages of the country’s civil war.

The travel restriction on the general is the first significant international penalty to be imposed on a Sri Lankan official over atrocities committed during the country’s 26-year civil war with Tamil Tiger militants, which ground to a halt in 2009. The United Nations estimates that up to 40,000 Tamil civilians died in those final stages, many in extrajudicial killings, accusations the Sri Lankan government has denied.

The move was a rebuke to a Sri Lankan government that is stacked with officials accused of rights abuses during the civil war — including Gotabaya Rajapaksa, who won presidential elections last November. Mr. Rajapaksa was the defense secretary during the final years of the war, and has appointed or promoted several officials accused of war crimes to important positions.

Now, observers and diplomats fear Sri Lanka is slipping back into illiberal democracy. Since Mr. Rajapaksa became president, activists, journalists and opposition members have increasingly come under pressure from the government, which has harassed them through the courts and police investigations and used pro-government media outlets to hound anyone suspected of dissent.