A senior State Department official says the United States has no plans to withdraw troops from South Korea even though President Reagan says it might be possible to reduce the U.S. military presence there.

"We have no intention of anything of that sort," Gaston Sigur, assistant secretary of state for East Asian and Pacific affairs, told reporters Thursday after Reagan met with South Korean President Roh Tae-woo."We would have to have a firm belief on the part of our country and the Republic of Korea that the threat from North Korea was over," Sigur said. "That threat is not over. We have certainly no intention of changing our policy of maintaining our ground forces in South Korea."

The president, posing for pictures with Roh in the Rose Garden, was asked by a reporter if he could "envision a time in the near future when the U.S. would be reducing its military presence."

"That is a possibility, but it would not be one of us just withdrawing - it would be one of (being) no longer needed," Reagan said.

The United States has about 42,000 troops stationed in South Korea under a mutual defense pact.

Sigur said the two presidents did not discuss the subject of troop reduction.

He said Reagan told Roh that the United States "fully supports" his proposals to bring about "a new atmosphere of reconciliation on the Korean peninsula."

The two presidents discussed several steps the United States is considering, Sigur said, adding that those steps would be announced "within the next few days or so."

He said these steps would not including lifting a U.S. ban on trade with North Korea, but could include "humanitarian trade . . . humanitarian aspects of relationships." He declined to elaborate.

Sigur said U.S. officials "feel comfortable" with Roh's proposal for a six-power conference including the United States, the Soviet Union, China, Japan and North and South Korea.

"This is his initiative and it's his to pursue," the State Department official said. "As I say, we can see some merit in it, but we have no intention of pushing it at this time."