Speaking to the US House of Representatives' Armed Services Committee on Thursday, General Burwell Bell said the solid fuel missiles tested were more reliable and easier to move around than anything the North has produced before.
Solid fuel missiles are also more durable and dramatically more accurate than liquid fuel missiles, he said.
US and South Korean officials said North Korea test-fired two short-range missiles on Wednesday, unsettling its Asian neighbors.
The tests came came during a stalemate in six-country talks aimed at ending North Korea's nuclear weapons programmes.
In addition, Bell said, the North was developing longer-range missiles with ranges "far beyond any requirement that they have for defence".
He said that with Pyongyang ready and willing to sell its missile technology to anyone willing to pay, there was a great concern about proliferation
Bell's comments contrasted with those of Admiral William Fallon, head of the U.S. Pacific Command, who told reporters on Thursday the North's missiles could reach targets in South Korea, but not beyond.
He said the North may have test-fired up to three solid fuel missiles, but he cautioned that he had not yet seen a detailed report.
During the hearing, Bell told members of congress that while the North appeared to be concentrating on its short-range missile program, estimates showed North Korea had the technological capability to develop missiles that "could reach throughout the continental United States."
"I am absolutely convinced, in a very strong way, that we have what we need on the peninsula to defeat an attack by North Korea ... quickly"
General Burwell Bell.
commander of US forces in South Korea
Bell also told lawmakers he was confident his 30,000 troops, together with South Korean soldiers, could turn back a North Korean invasion.
"If the North miscalculated, I am absolutely convinced, in a very strong way, that we have what we need on the peninsula to defeat an attack by North Korea ... quickly," Bell said.
Bell was responding to a hypothetical scenario presented by Democratic Representative Gene Taylor, who asked how Bell's troops would fare if, simultaneously, China attacked Taiwan, North Korea attacked the South and a terrorist blew up levees in New Orleans, diverting US reinforcements and supplies from the Korean Peninsula.
Bell acknowledged that US operations in Iraq, Afghanistan and the massive cleanup from Hurricane Katrina made it necessary for the United States to make sure US troops in other parts of the world were well-supplied. But he said he was confident his troops had enough training and materiel.
He declined to say at a public hearing how long he thought his troops could last, under Taylor's scenario.
He said, however, that the number of days was the same today as it was four years ago, because the US force was smaller and the South Korean troops more competent.