"While the elections were calm and peaceful and largely without any kind of violence, we note with some degree of remorse that the elections there were not up to international standards," Assistant Secretary of State Johnnie Carson told a House of Representatives panel.

Carson, the Obama administration's top diplomat for Africa, said Ethiopian Prime Minister Meles Zenawi's government had taken "clear and decisive" steps to ensure it won a landslide victory in Sunday's vote. The ruling Ethiopian People's Revolutionary Democratic Front and allied parties won nearly every seat in the country's 547-member parliament.

A European Union observer mission said on Tuesday the polls had been marred by reports of violence and intimidation, as well as the ruling party's use of state resources for campaigning, but that this did not invalidate the results.

"It is important that Ethiopia move forward in strengthening its democratic institutions and when elections are held that it level the playing field to give everyone a free opportunity to participate without fear or favor," Carson said.

The United States regards Ethiopia as a crucial ally in the fight against hardline Islamism in the Horn of Africa, and has provided some $4.7 billion in aid to the country between 1999-2009, including $862 million in 2009, according to State Department figures.

But Washington has also noted opposition accusations of repression by Meles, who has been in power since 1991.

The two countries traded sharp words in March after Meles accused the U.S.-funded Voice of America broadcaster of spreading "destabilizing propaganda" and said it would explore jamming the broadcaster's Amharic language service.

Carson indicated that the United States would continue to press Meles to make democratic changes, but not at the price of endangering the alliance.

"We appreciate the level of collaboration that we receive from Ethiopia in a number of areas, but we also believe that Ethiopia must do better in strengthening its democratic institutions," he said.

(Reporting by Andrew Quinn; editing by Jackie Frank)