The United States on Wednesday voiced concern about Malaysia\'s weekend crackdown on an opposition-backed rally and said it would keep an eye on developments. Riot police fired tear gas and water cannon to end Saturday\'s electoral reform rally and arrested more than 1,600 people. One man died during the rally. His family said he collapsed after fleeing tear gas, while police said he was a bystander who died of a heart attack. \"We do have some concerns,\" State Department spokesman Mark Toner told reporters. \"We... continue to stand for the right for people to freely express their democratic aspirations and express their views freely.\" \"I would stress that those must be peaceful demonstrations,\" Toner said. \"We continue to monitor the situation closely.\" Malaysia has promised to investigate allegations of police brutality. Prime Minister Najib Razak has defended the police, saying the rally was a ploy to tarnish the country\'s image. International human rights groups strongly criticized the crackdown and urged the United States and other countries to put pressure on Malaysia to ensure accountability. \"The Malaysian government should launch a prompt, impartial, and transparent investigation into the use of excessive force and unwarranted arrests by the security forces,\" New York-based Human Rights Watch said in a statement. Malaysia has been looking to build closer relations with the United States amid an effort by President Barack Obama\'s administration to reach out to Southeast Asia. Malaysia and the United States have long been major trading partners but political relations have been uneasy, particularly during the 22-year-rule of Mahathir Mohamad -- an outspoken critic of US foreign policy. The United States has also been concerned about the treatment of opposition leader Anwar Ibrahim who spent six years in jail and is again on trial on allegations of sodomy, which is illegal in the Muslim-majority nation. Anwar says that the charges are politically motivated. Obama invited Najib to Washington in April 2010 for a major summit on nuclear security and met him on the sidelines. Secretary of State Hillary Clinton visited Malaysia in November