The Pentagon warned Pyongyang on Friday that "further provocative action would be regrettable" after reports that North Korea had deployed two mid-range missiles near its eastern coast.
"Missile tests outside their international obligations would be a provocative act. They need to follow international norms and abide by their commitments," Pentagon spokesman George Little told reporters.
Several UN Security Council resolutions require North Korea to abstain from all nuclear and ballistic missile activities.
Little declined to comment on a report by South Korea's Yonhap news agency saying that two intermediate Musudan missiles had been transported by train earlier in the week and loaded on vehicles equipped with launch pads.
The Musudan has never been tested, but is believed to have a range of around 3,000 kilometers (1,860 miles), which could theoretically be pushed to 4,000 if they were to be given a light payload.
That would cover any target in South Korea and Japan, and possibly even reach US military bases located on the Pacific island of Guam.
The Pentagon has said it will send missile-interceptor batteries to protect its bases on Guam, a US territory some 3,380 kilometers (2,100 miles) southeast of North Korea and home to 6,000 American military personnel.
Most experts think the North is not yet capable of mounting a nuclear device on a ballistic missile which could strike US bases or territory.
Tensions have soared on the Korean peninsula since December, when the North test-launched a long-range rocket. In February, it conducted its third nuclear test and drew fresh UN sanctions.
The North also warned this week it would reopen its mothballed Yongbyon reactor -- its source of weapons-grade plutonium that was closed in 2007 under an aid-for-disarmament accord.
Little defended the current South Korean-US military drill which has infuriated Pyongyang, saying "we have been responsible and prudent in how we (have) conducted these exercises," which run through April 30.
"The North Koreans on their side need to ratchet the rhetoric down, which has been bellicose, overheated and unproductive," he said.