WASHINGTON (REUTERS) - The United States sent its top Asian diplomacy and security officials to South Korea and Japan to calm tensions between two US allies whose squabbling has frustrated efforts to deal with a troublesome North Korea and an increasingly assertive China.

The high-powered delegation from the White House, Pentagon and State Department departed on Monday and will be visiting the region shortly after the election of a new nationalist-leaning Japanese government in December and before Seoul inaugurates anew president in February.

Washington hopes South Korea and Japan can put a lid on spats over history and territory stemming from Japan's 1910-1945 occupation of Korea. US officials also seek to reassure Tokyo as it confronts almost daily challenges from China over which has sovereignty of disputed islets in a separate, more dangerous, territorial row with Beijing.

The long-simmering disputes erupted anew last year, plunging Tokyo's ties with Seoul and Beijing to troubling lows and casting a cloud over the President Barack Obama's signature policy for East Asia - rebalancing security forces in the region - in part to cope with a surging China.

"We want to see the new Japanese government, the new South Korean government, all of the countries in Northeast Asia working together and solving any outstanding issues, whether they are territorial, whether they're historic, through dialogue," State Department spokesman Victoria Nuland said last week.

Troubles between Asia's second and fourth biggest economies are frustrating to Washington at a time when a defiant North Korea has tested a long-range rocket and may be poised to conduct its third nuclear test.

In one of the final acts before Obama brings in a new national security team for his second term, Assistant Secretary of State Kurt Campbell, Assistant Secretary of Defense Mark Lippert and Daniel Russel, the National Security Council senior director for Asian affairs, will meet officials in Seoul and Tokyo.

US officials regularly meet counterparts from the two countries, which have been American allies since the 1950s and together host most of the 80,000 US troops in Asia. But the antagonistic nationalism that flared up in Asian capitals last year makes this trip anything but routine.

Another understated aim of the US mission to Tokyo this week is to convince Japanese Prime Minister Shinzo Abe to step away from some of the more nationalist policies of the platform on which he campaigned and won office on last month.

Washington is particularly concerned about MR Abe's previous calls to revise or rescind a landmark 1995 apology for Japan's wartime aggression and 1993 statement acknowledging an official Japanese role in the recruiting of tens of thousands of mostly South Korean "comfort women" to serve troops during World War Two.

Such actions would anger Asian nations that suffered from Japan's militarism, further complicating both US attempts to manage ties between its allies in the region and relations with China, which also is ushering in new leadership in March.

Chief Cabinet Secretary Yoshihide Suga said last week that MR Abe would stand by the 1995 apology. Although Mr Abe packed his Cabinet with politicians who hold extremely revisionist views of history, analysts are predicting policies will be pragmatic, with a focus on reviving the economy.

"The Abe administration basically will not touch foreign diplomacy and security affairs before the Upper House election" in July, former Vice Minister for Defence Motohiro Oono of the opposition Democratic Party of Japan told a think tank panel in Washington last week.

Mr Bruce Klingner, an analyst at the conservative Heritage Foundation, said recent statements from Abe have been "suitably nuanced." He said during Mr Abe's 2006-2007 tenure as prime minister, he "defied many of the same predictions by maintaining and even improving Japan's relations with its neighbours."

Mr Abe's gestures to neighbours include sending an envoy to meet South Korean President-elect Park Geun Hye; announcing that his first overseas trip since winning office will be to Indonesia, Vietnam and Thailand; and offering to supply the Philippines with 10 coast guard vessels and communications equipment to help Manila in its territorial dispute with China.