"The United States is concerned that this constitutional amendment weakens checks and balances and thus undermines the principles of constitutional democracy," State Department spokesman P.J. Crowley said in a written statement.
Sri Lanka's national parliament on Wednesday voted in favour of the 18th amendment to the country's constitution, scrapping a two-term limit on the presidency and widening President Mahinda Rajapakse's control of the state.
The president's office said Rajapakse was "pleased" with the parliamentary endorsement, which is another success for the 64-year-old populist leader after a string of recent electoral victories.
As well as allowing him to stand in the 2016 polls, the amendment also hands Rajapakse greater control over previously quasi-independent institutions such as the judiciary, police, election commission and central bank.
Crowley said the United States was calling on the government of Sri Lanka to promote the principles of good governance, democra