WASHINGTON (Yonhap) -- North Korea may have already developed nuclear warheads that are small enough to be mounted on missiles and aircraft, a senior U.S. intelligence official said Thursday.

   "The North may now have several plutonium-based nuclear warheads that it can deliver by ballistic missiles and aircraft as well as by conventional means," Lt. Gen. Ronald Burgess, director of the Defense Intelligence Agency, told a Senate Armed Services Committee hearing. "We expect the North will continue to test-launch missiles, including the TD-2 ICBM/SLV to refine their performance. With further TD-2 tests, North Korea may develop an intercontinental ballistic missile capable of reaching the U.S. mainland."

   North Korea detonated nuclear devices in 2006 and 2009, and conducted long-range missile tests three times -- in 1998, 2006 and 2009 -- which were seen as a partial success.

   Reports said that North Korea is digging a new tunnel to prepare for a third nuclear test and has completed construction of a sophisticated launch site on its western coast to test-fire a ballistic missile that can reach the mainland U.S.
   Defense Secretary Robert Gates said in January that North Korea's missiles and nuclear weapons will pose a threat to the U.S. within five years, with some experts saying Pyongyang may have already developed nuclear warheads small enough for missile payloads.

   Burgess said North Korea will not easily abandon its nuclear arsenal despite international efforts to denuclearize the impoverished, but nuclear-armed communist state.

   "While North Korea may be willing to abandon portions of its nuclear program in exchange for improved relations with the United States, Pyongyang is unlikely to eliminate its nuclear weapons," he said. "The DPRK will try to keep its nuclear weapons and gain international recognition as a nuclear state together with security guarantees from Washington and expanded economic assistance."

   The six-party talks on the North's nuclear dismantlement have been deadlocked over U.N. sanctions for Pyongyang's nuclear and missile tests in early 2009, and most recently the North's torpedoing of a South Korean warship and shelling of a South Korean border island last year.

   North Korea last year attacked the warship Cheonan and Yeonpyeong Island, killing 50 people, chilling inter-Korean ties to the lowest level since the end of the 1950-53 Korean War.

   North Korea refused to apologize for the provocations and walked out of a rare inter-Korean dialogue last month, thwarting hopes for the resumption of the six-party talks.

   Washington has called on Pyongyang to address Seoul's grievances over the deadly attacks before moving on to a new round of the multilateral denuclearization-for-aid talks.

   James Clapper, national intelligence director, buttressed Burgess.

   "Iran and North Korea are of great concern," Clapper said. "In time they pose a direct mortal threat to the continental United States."

   Clapper said North Korea sees its nuclear arsenal as its only leverage.

   "Obviously they continue to play their nuclear card," he said.

"That is their single, I think, leverage point, or leverage device they can use to attract attention and seek recognition for them as a nuclear power."

   The U.S. spy chief also warned of further provocations from the North. 
   "It is also our assessment at this time that there is a low probability of a conventional attack by the North upon the South,"

Clapper told the hearing, but added, "North Korea has shown a proclivity for doing sometimes the unexpected and it is the unintended consequences of those events that may precipitate something else."

   Burgess said that North Korean leader Kim Jong-il's plans to transfer power to his youngest son, Jong-un, explain recent provocations.

   "Of significant concern is decision making relative to the apparent leadership succession under way and its implications for additional deliberate provocations against the South," Burgess said. "The North Korean artillery attack against Yeonpyeong Island on Nov. 23, 2010, and torpedo attack on the naval corvette Cheonan on March 26, 2010, show Pyongyang's willingness to use military force to advance his external and internal goals. Miscalculation could lead to escalation."

   The recent provocations are seen as part of the 28-year-old heir's effort to rally support from the military, just like his ailing father is suspected of masterminding the downing of a Korean Air plane in 1987 that killed all 115 passengers aboard.

   Kim Jong-il was being groomed at the time to succeed his father and North Korea's founder, Kim Il-sung, who died of a heart attack in 1994.

   Unlike Kim Il-sung, who was a guerrilla leader against Japanese colonialists, neither Kim Jong-il nor Jong-un has a proper background in the military, which serves as a linchpin for the impoverished communist state.

   North Korea, meanwhile, disclosed in November a uranium enrichment plant that could be used to make nuclear weapons apart from its plutonium program. The North claims its intention is to generate electricity.

   South Korea and the U.S. have said they will seek a U.N. Security Council presidential statement to condemn the uranium program before moving on to the nuclear talks involving the two Koreas, the U.S., China, Japan and Russia.

   China, North Korea's staunchest communist ally, wants the uranium issue to be dealt with at the six-party talks and opposes Security Council involvement, citing a possible adverse impact on an early resumption of the nuclear talks.

   North Korea, suffering severe food shortages from a bad harvest, floods and cold weather last year, recently asked for massive food aid from the international community. Washington is assessing that request.

   Reports have said talks toward another inter-Korean summit have been deadlocked over Pyongyang's demand for hundreds of tons of food aid as a precondition without any commitment to address South Korea's grievances over last year's provocations.

   Rep. Ileana Ros-Lehtinen (R-FL), chairman of the House Foreign Affairs Committee, opposed any food aid to North Korea unless the U.S. secures transparency in the distribution of food aid.

   "Pyongyang has requested further U.S. food aid as reports indicate renewed food shortages in North Korea," Ros-Lehtinen said.

"There is the question of the American food aid remaining in North Korean warehouses when Pyongyang expelled American humanitarian NGOs in the spring of 2009. Pyongyang distributed this food, without monitoring."

   U.S. food aid to the North was suspended in early 2009 amid heightened tensions over Pyongyang's nuclear and missile tests and controversy over the transparency of food distribution.

   North Korea at the time refused to issue visas to Korean-speaking monitors, whose mission was to assure that the food was not funneled to the military and government elite.

   A joint team from the World Food Program and the Food and Agriculture Organization is currently in North Korea to assess the country's food situation.

   "There must be a full accounting of these 20,000 tons of food aid requested," she said. "Lest we forget, in December 2008, U.S. shipments of food aid to North Korea via the World Food Program were suspended due to growing concerns about diversion to the North Korean military and regime elite and WFP's lack of effective monitoring and safeguards. Fast approaching is the 100th anniversary next year of the birth of Kim Jong-il's father, and there is the danger that aid provided would be diverted for this spectacle."