North Korea threatens nuclear strike over US carrier arrival in South Korea

USS Ronald Reagan aircraft carrier arrived in South Korea as part of agreement to increase US military presence.
North Korea denounced the carrier's arrival as a provocation and warns of catastrophic consequences.
North Korea threatens to use nuclear weapons if it perceives an imminent threat.
Concerns raised about North Korea seeking advanced weapons from Russia.
Experts say North Korea heightens tensions to expand nuclear arsenal and gain concessions.