North Korea conducts third test of rocket engine this month

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US officials believe North Korea nuclear test is imminent

Secretary of state says all options are on the table

North Korea tested a rocket engine for the third time this month -- and like the previous two tests, it showed signs the rogue nation's missile capabilities were improving, a U.S. official confirmed Tuesday to Fox News.

None of the three rocket tests ended in an embarrassing explosion, showing the secretive regime has made progress, the official said. The test did not demonstrate any new capabilities.

The Pentagon continues to see signs North Korea is edging closer to conducting another nuclear test, following two successful tests last year.

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North Korea conducted the latest test last Friday night, Reuters reported, adding that Kim Jong Un's regime could use the engine in an intercontinental ballistic missile, or ICBM.

Kim claimed earlier this month that a high-thrust engine test symbolized a "new birth" of his country's rocket industry.

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Meanwhile, a nuclear-powered U.S. Navy fast-attack submarine joined the Pentagon's annual joint military exercises with South Korea. "The U.S. Navy's USS Columbus (SSN-762) is participating in the Foal Eagle exercise," a defense official told the Yonhap news agency on Tuesday.

The USS Carl Vinson aircraft carrier and at least two Air Force bombers already had joined the military drills. Secretarty of State Rex Tillerson said earlier this month on a visit to South Korea that the Obama-era policy of strategic patience was over.

North Korea is pushing hard to upgrade its weapons systems, analysts say. Many weapons experts warn the North could have a functioning nuclear-tipped missile capable of reaching the continental U.S. within a few years.

North Korea carried out two nuclear tests last year.

U.S. defense officials had said last week that the Pentagon was expecting another North Korean missile launch within days. The officials said the U.S. had increased its surveillance over the North and had detected a North Korean missile launcher being moved, as well as the construction of VIP seating in Wonsan.

Washington, Seoul and others view North Korea's rocket program as a cover for its banned long-range missile development program.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.