A series of press reports in Seoul said the allies have reached a deal to allow South Korea to develop ballistic missiles with the range of up to 800 kilometers, enough to cover all of North Korea.
South Korea is currently prohibited from developing ballistic missiles with the range of more than 300 km.
There will be no change in the 500-kg limit on payload weight under envisioned new guidelines, reports said.
Multiple sources said the two sides plan to make an announcement on their agreement very soon.
The State Department neither confirmed nor denied that.
"As you know, we continue to talk about bilateral security arrangements. We've made that clear here," department spokeswoman Victoria Nuland said at a press briefing. "I don't have anything new to report to you today on the missile side."
The White House also refused to respond to Yonhap News Agency's separate inquiry.
The missile range issue is highly sensitive for South Korea and the U.S. alike.
In its final months in office, South Korea's conservative Lee Myung-bak administration hopes to present a ticket for longer-range missiles as a diplomatic achievement.
It could help solidify the conservative base ahead of the presidential polls in December.
Lee is banned from seeking re-election himself under the Constitution, but he hopes his ruling party will retain power.
For the U.S., Seoul's push raises concerns over a regional arms race.
"From the perspective of the U.S., it is not only a bilateral matter but also associated with global nonproliferation issues," a diplomatic source.
The State Department also earlier told Yonhap, "We must consider this issue responsibly and ensure that any steps we take to ensure strong Republic of Korea defense capabilities are consistent with our shared regional and global nonproliferation objectives."