US military commanders are offering a strident defense of plans to give South Korea control of both nations' forces in two years, brushing aside concerns it could worsen the threat from North Korea.

As of April 17, 2012, South Korea is set to be in control of both its own and US forces in the event of war. South Korea has already had peacetime operational control since 1994, but with a US general taking over in wartime.

Admiral Robert Willard, head of the US Pacific Command which covers Asia, testified before Congress on Friday that he was impressed by South Korea's capabilities during military exercises.

"We're convinced that operational control transition could clearly occur in 2012," Willard told the Senate Armed Services Committee.

Appearing beside him, General Walter Sharp, commander of the 28,500 US troops in South Korea, said that a shift in command was "the right thing to do."

"The number one responsibility of any nation is to defend their own country," he said, particularly "a country that is as advanced as Korea is."

"It also sends a very strong message to North Korea and to other people in the region that the Korean military is so strong that the US is willing to go in a supportive relationship," he said.

A delay in the transfer would send "exactly the opposite signal, which is not the right thing to do," he said.

But analysts and policymakers in both countries have increasingly questioned the transfer as tensions remain rife with nuclear-armed North Korea.

In South Korea, supporters of delaying the handover say they have collected millions of signatures from former military members.

Senator Joe Lieberman, a hawkish independent senator from Connecticut, voiced concern about making the transition in a year as busy as 2012.

The United States and South Korea both hold presidential elections in 2012 and North Korea may make major announcements as it celebrates the 100th birth anniversary of its late founder and "eternal president" Kim Il-Sung.

Lieberman voiced fear that the switch would have an "impact on other areas of the world, including particularly Afghanistan and Pakistan, where they worry about whether we're going to leave before the job is done."

The United States has a security alliance with Seoul dating from the 1950-53 Korean War which ended with an armistice but no peace treaty, and left the peninsula divided.

The transfer agreement was reached in 2007 under liberal former South Korean president Roh Moo-Hyun. Relations with the United States have improved markedly since conservative Lee Myung-Bak assumed the presidency in 2008.

Hwang Jin-Ha, a member of Lee's Grand National Party, told a symposium in Washington that the two nations had reached the agreement "to pursue their own national interests based on their strategic misunderstandings."

Hwang said the Roh administration cast the issue as one of national pride, while former US president George W. Bush's team was eager to avoid a South Korean backlash and to give US forces more freedom of movement as part of a global realignment.

"It was as if two trains bound for different final destinations met accidentally at an unscheduled station and then continued on to their original destinations," Hwang said.

But analysts doubted that President Barack Obama's administration would be eager to cast the 2007 agreement as flawed.

The Obama administration is pressing Japan's six-month-old left-leaning government to stick to a separate troop plan, arguing that countries must stay true to agreements despite changes in administration.

Scott Snyder, director of the Center for US-Korea Policy at the Asia Foundation, said the solution may be to put less emphasis on the command transfer and instead frame it as one detail in the nations' plans for greater cooperation.

"Such a way avoids the prospect of a backlash," Snyder said.

"You definitely don't want this to become another beef issue," he said, referring to mass protests in 2008 over beef imports.