"The Sri Lankan people deserve better. In fact, as the government of Sri Lanka fails to implement LLRC recommendations, the outlook for human and political rights in Sri Lanka appears to be getting worse," the senators wrote in a letter to outgoing Secretary of State Hillary Clinton.
The senators voiced concern over the recent impeachment, backed by President Mahinda Rajapakse, of the chief justice and highlighted charges of human rights groups that authorities have intimidated the media.
"Accountability is a necessary precursor to reconciliation and a stable democracy in Sri Lanka," the senators wrote.
"It is clear to us that the LLRC process is mired in bias and delays, and only an independent, international investigation will achieve real accountability," they wrote.Sri Lanka has been accused of killing as many as 40,000 civilians in 2009 in a final offensive that defeated the Tamil Tigers, who had waged a decades-long insurgency and were also criticized for tactics such as suicide bombings.
Sri Lanka has denied that any civilians were killed and rejected an international probe.
The Obama administration has stopped short of supporting human rights groups' calls for an international probe in Sri Lanka but has gradually lost patience with Rajapakse.
US officials announced Monday in Colombo that the United States for the second straight year will bring a resolution before the UN Human Rights Council to pressure Sri Lanka over its record.