BEIJING (Reuters) - China reiterated Monday that the United States' arms sales to Taiwan seriously undermines its core interests and would harm ties between Washington and Beijing.

China's Vice Foreign Minister Cui Tiankai made the comments to reporters during a briefing, weeks after the United States said it would sell $5.85 billion in military hardware to the island China calls a breakaway province.

U.S. arms sales to Taiwan, a self-ruled island that China says eventually must unify with the mainland, have been a persistent irritant in Sino-U.S. ties, adding to economic strains between Washington and Beijing.

The Obama administration said last week that it is weighing fresh arms sales to Taiwan as part of a sweeping effort to deter any Chinese attack on the island.

Such supplies would be on top of plans sent to Congress on September 21 to sell new hardware and defense services, including upgrades for Taiwan's 145 F-16 A/B fighter aircraft, bought in 1992.

China's People's Liberation Army had previously said the U.S. offer to Taiwan -- which also includes advanced air-to-air missiles, laser- and GPS-guided bombs and radars -- could disrupt relations between the two countries' militaries.

The PLA suspended ties with the U.S. military for most of 2010 after a previous $6.4 billion U.S. arms package for Taiwan.

(Reporting by Michael Martina, Writing by Sui-Lee Wee; Editing by Ken Wills)