U.S. officials are closely monitoring the situation in North Korea, but said Thursday there are "no signs of an imminent long-range launch" of a long-range missile headed toward Hawaii.

But officials warned of the possibility of a short- or medium-range missile launch, saying the North Koreans could "rack one up" in a matter of hours.

North Korea has vowed to enlarge its nuclear arsenal and threatened the U.S. with "annihilation." But senior U.S. officials are putting the ramped up rhetoric as part of a "continuing North Korea bluster" designed to coincide with the 59th anniversary of the outbreak of the Korean War.

The North Koreans are sensitive to anniversaries and would wish to exploit this symbolic date for domestic purposes.

Tens of thousands of North Koreans joined a rally in Pyongyang on Thursday to shout slogans to denounce international sanctions and mark the anniversary of the start of the Korean War that split the peninsula into the communist North and capitalist South. 

The rally followed President Obama's decision Wednesday to extend U.S. economic sanctions against North Korea for another year, saying the North's possession of "weapons-usable fissile material" and its proliferation risk "continue to pose an unusual and extraordinary threat" to the United States.

An editorial in a major Japanese newspaper last week speculated that the launch of a Taepodong-2 ballistic missile, able to reach Hawaii, was possible in early July, and had been timed possibly to coincide with the July 4th celebration. 

The missile is thought to be capable of carrying a nuclear warhead. But U.S. officials say they aren't seeing the activity on the ground that they expect would be used for a Taepodong-2 launch.

The Pentagon said interceptors in California and Alaska are ready to deter any incoming threats from North Korea. Missile defense systems have also been deployed in Hawaii.

Defense Secretary Robert Gates said last week that additional assets were being deployed to shore up the defenses of the Hawaiian Islands.

On Wednesday, North Korea announced maritime restrictions, prohibiting international sailing off its east coast beginning Thursday through July 10. The announcement is considered another marker that North Korea may be close to either military exercises or a short- or medium-range missile launch.

FOX News' Catherine Herridge and The Associated Press contributed to this report.