General Sharp said all available options were open
The commander of US forces in South Korea has said he has not ruled out a military response if North Korea test-fires a long-range missile.
US General Walter Sharp urged North Korea to cease what he referred to as provocations, and behave like a responsible country.
North Korea is reported to be preparing to test-fire a long-range missile.
South Korea's President, Lee Myung-Bak, has also said he will stand firm in the face of threats from the North.
Tension in the Korean peninsula has risen in recent weeks - with the North warning that it considers itself on the brink of war with the South.
According to the South Korean media, recent satellite spy photos show a large cylindrical object being moved towards North Korea's long-range missile site.
North Korea carried out a "failed" Taepodong-2 missile test in 2006
Speaking at a press conference for foreign journalists, General Walter Sharp - the commander of the more than 28,000 American troops stationed in South Korea - said he would not confirm what is, or is not, known by intelligence sources.
But when asked what he would do if North Korea was to proceed with a long-range missile test, he said: "We have all available options open to us".
"That includes everything from diplomatic to economic sanctions, to military options," he said.
"Of course we very much want the military option to be the last to be taken but those options are all open to us."
North Korea's last long-range missile test in 2006 was met with international condemnation and UN sanctions, but no military action.
Dec 07: Lee Myung-bak wins South Korean presidential election. Vows tougher line on the North
March 08: North expels S Koreans from joint industrial park after Seoul says it will link its aid more closely to the nuclear disarmament issue
April 08: N Korean media warns President Lee his tough stance could have "catastrophic consequences"
July 08: Pyongyang rejects President Lee's offer of direct talks
Oct 08: Military officials from both sides hold first direct talks since President Lee took office
Nov 08: N Korea says it will close land borders, suspend tourism trips and the joint train service because of "relentless confrontation" from Seoul
Dec 08: N Korea enforces stricter border controls and expels hundreds of South Koreans from the joint industrial zone
Jan 09: The North says it is scrapping all military and political pacts signed with the South
Diplomatic tension has been rising in recent weeks, with increasingly angry rhetoric coming from North Korea, warning that relations with the South have reached what it called "the brink of war".
Some analysts believe this is mere posturing as an attempt to strengthen its negotiating position with the new US administration.
But inter-Korean relations have steadily deteriorated significantly since the conservative Mr Lee took office in Seoul in February last year.
Mr Lee said on Monday that though his ministers were always ready to talk with the North, they would not rush into dialogue.
"We are ready to work with North Korea. North Korea, too, must realise the South is the only country in the entire world that is sincerely concerned for its future and willing to help it," Mr Lee said.
His comments came ahead of a confirmation hearing for his new Unification Minister, Hyun In-Taek, who North Korea has denounced as an "anti-DPRK [North Korea] confrontational fanatic and sycophantic traitor serving the US".
North Korea said if Mr Hyun were confirmed in the job, inter-Korean relations would collapse into ruin.