South Korea's military went on heightened alert Thursday as concerns arose that North Korea could use the distraction of war in Iraq to raise tensions on the Korean Peninsula.

"We expect North Korea to be cautious, but we have strengthened our alert status and our early-warning status in response to possible North Korean attempts to increase tensions," presidential spokeswoman Song Kyoung-hee said, without elaborating.

In a televised address, South Korean President Roh Moo-hyun called the war in Iraq "an inevitable measure to eliminate weapons of mass destruction as quickly as possible, at a time when diplomatic efforts to resolve the issue peacefully have failed."

Roh vowed to ensure that the war will not worsen the situation on the Korean Peninsula.

After presiding over a National Security Council meeting, Roh said his military strengthened its vigilance and national police would beef up security to guard against possible terrorist attacks.

However, the Defense Ministry denied initial reports that the alert level was the highest in seven years.

On Wednesday, the U.S.-led United Nations Command sought to ease North Korean fears over joint military exercises in South Korea, saying they are defensive and not related to "current world events."

The North has maintained the exercises signal plans to invade it. Pyongyang refused a request to discuss the matter at a higher military level on Thursday, the U.N. Command said.

Meanwhile, the U.S. military in South Korea announced plans to implement a new curfew beginning late Thursday.

"The new curfew is aimed at protecting U.S. soldiers and civilian employees from anybody that might want to potentially use the world situation to their benefits," said Lt. Col. Steven Boylan, a spokesman for the U.S. Eighth Army.

All 37,000 U.S. soldiers stationed in South Korea must be off the streets by 7:30 p.m., several hours earlier than the normal curfew, he said.

Referring to the North's refusal to agree to a high-level meeting on the joint exercises, Col. Martin Glasser of the Command's Military Armistice Commission said the North "has turned down an excellent opportunity to discuss important events affecting the Korean peninsula."

Glasser said the annual exercises are not related to "current world events."

"We also explained that the exercise is defensive in nature and is not an aggressive or a threatening move against North Korea," he said in a statement. "And that these are regularly scheduled exercises much like the exercises they routinely conduct in North Korea."

North Korea insisted the United States is preparing to attack, a claim it has made in previous years.

"The ever more reckless saber rattling of the U.S. imperialists is, in a nutshell, a premeditated move to mount a pre-emptive nuclear attack," the official newspaper, Rodong Sinmun, said Wednesday.

Military exercises began early this month and will continue through April 2.

Also Wednesday, North Korea said it has the right to develop missiles, increasing fears it might resume test-launching long-range missiles.

Pyongyang has fired two short-range missiles off its east coast in recent weeks, raising tensions in a region already roiled by a standoff over its suspected nuclear weapons programs.

North Korea insisted its missile program "is of purely peaceful nature and does not pose a threat to anyone," in a commentary carried by the North's official news agency KCNA. The North, it said, has a "sovereign right to go ahead with its missile program."

Japanese media reported last week that North Korea appeared to be making final preparations to test-launch a ballistic missile, although government officials in the region have denied having strong evidence that a test is imminent.

With the United States focused on Iraq, experts say North Korea might use the opportunity to test long-range missiles or reprocess spent nuclear fuel to make atomic bombs. That would be viewed as an attempt to force Washington into direct negotiations over its nuclear programs.

The Korean nuclear crisis flared in October, when U.S. officials said Pyongyang admitted having a uranium program. Washington and its allies suspended fuel shipments, and the North retaliated by expelling U.N. monitors, withdrawing from the Nuclear Nonproliferation Treaty and restarting a nuclear reactor.