A key U.S. senator says the United States is pleased with recent progress toward democratic reforms in Burma, but says the country's new, nominally civilian government must ensure “free and fair” by-elections in April.

John McCain, the senior member of the Senate Armed Services Committee, spoke Sunday in Rangoon, after meeting with President Thein Sein and separately with opposition leader and democracy advocate Aung San Suu Kyi.

“We are very encouraged by recent events but we also recognize that there is a great deal to be done.”

He said he told the president the government must also resolve Burma's long standing ethnic conflicts and strengthen the rule of law before Washington would consider lifting economic sanctions imposed on the military junta that stepped aside last year.

On Saturday, the U.S. delegation, which also includes Senator Joseph Lieberman, stepped up pressure on Vietnam to improve its human rights record if it wants to expand bilateral military ties with Washington.

McCain cited “backward movement” by the Hanoi government on human rights issues and warned that bilateral security ties will be “directly impacted” by Hanoi's failure to improve its rights record.

Lieberman cited Vietnam's interest in purchasing U.S. weapons systems, but said such sales are not going to happen unless Washington sees human rights advances in the country.

Vietnam has been seeking to expand military ties with the United States as it faces ongoing regional tensions, especially with China, over competing maritime claims in the South China Sea.