SEOUL (Reuters) - North Korean leader Kim Jong Un said his country has miniaturized nuclear warheads to mount on ballistic missiles and ordered improvements in the power and precision of its arsenal, state media reported on Wednesday.

Kim has called for his military to be prepared to mount pre-emptive attacks against the United States and South Korea and stand ready to use nuclear weapons, stepping up belligerent rhetoric after coming under new U.N. and bilateral sanctions last week for its nuclear and rocket tests.

U.S. and South Korean troops began large-scale military drills this week, which the North called “nuclear war moves” and threatened to respond with an all-out offensive.

Kim’s comments were his first direct mention of the claim, made repeatedly in state media, to have successfully miniaturized a nuclear warhead, which has been widely questioned and never independently verified.

“The nuclear warheads have been standardized to be fit for ballistic missiles by miniaturizing them,” KCNA quoted Kim as saying as he inspected the work of nuclear scientists, adding “this can be called a true nuclear deterrent”.

“He stressed the importance of building ever more powerful, precision and miniaturized nuclear weapons and their delivery means,” KCNA said.

Responding to the KCNA report, a U.S. State Department spokeswoman, Katina Adams, repeated a call on North Korea to “refrain from provocative actions and rhetoric that aggravate tensions.”

Kim also inspected the nuclear warhead designed for thermo-nuclear reaction, KCNA said, referring to a miniaturized hydrogen bomb that the country said it tested on Jan. 6.

Rodong Sinmun, official daily of North Korea’s ruling party, carried pictures of Kim in what seemed to be a large hangar speaking to aides standing in front of a silver spherical object.

They also showed a large object similar to the KN-08 intercontinental ballistic missile (ICBM) previously put on display at military parades, with Kim holding a half-smoked cigarette in one of the images.

South Korea’s defense ministry said after the release of the images that it did not believe the North has successfully miniaturized a nuclear warhead or deployed a functioning ICBM.

That assessment is in line with the views of South Korean and U.S. officials that the North has likely made some advances in trying to put a nuclear warhead on a missile, but that there is no proof it has mastered the technology.

Chinese Foreign Minister Wang Yi, speaking by telephone to U.S. Secretary of State John Kerry, described the situation on the Korean peninsula as “very tense” and called for all parties to remain calm and exercise restraint, China’s foreign ministry said.

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North Korea conducted its fourth nuclear test on Jan. 6 but its claim to have set off a miniaturized hydrogen bomb has been disputed by the U.S. and South Korean governments and many experts.

Following on from the U.N. sanctions, South Korea on Tuesday announced further measures aimed at isolating North Korea by blacklisting individuals and entities that it said were linked to Pyongyang’s weapons program.

China also stepped up pressure by barring a North Korean freighter from one of its ports.

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But a U.N. panel set up to monitor sanctions under an earlier Security Council resolution adopted in 2009 said in a report released on Tuesday that it had “serious questions about the efficacy of the current U.N. sanctions regime.”

North Korea has been “effective in evading sanctions” by continuing to engage in banned trade, “facilitated by the low level of implementation of Security Council resolutions by Member States,” the Panel of Experts said.

“The reasons are diverse, but include lack of political will, inadequate enabling legislation, lack of understanding of the resolutions and low prioritization,” it said.