WASHINGTON, Sept 21, 2011 (AFP) - The Obama administration came under fire Wednesday from both China and domestic critics as it approved a $5.85 billion upgrade of Taiwan's fighter jets that stopped short of selling new F-16s.

Taiwan and US officials said that the upgrade would improve the island's defenses as it faces a rising China, which has ramped up military spending and widened its strategic edge over the self-governing territory.

Officials in Washington and Taipei said Taiwan would get a retrofit of 145 F-16 A/B fighter jets, which will be equipped with modern weapons and radar capable of detecting China's new stealth airplanes.

State Department spokeswoman Victoria Nuland called the package a "substantial" move that will "help ensure that Taiwan maintains the capability to protect its airspace in both peacetime and during any crisis."

"We firmly believe that our arms sales to Taiwan contribute to the maintenance of peace and stability across the Taiwan Strait," she said.

But China, which claims Taiwan as its territory and has repeatedly warned the United States not to sell weapons to the island, summoned US Ambassador Gary Locke and warned the move "will inevitably undermine" relations including military ties.

"China strongly urges the United States to be fully aware of the high sensitivity and serious harm of the issue, seriously treat the solemn stance of China, honour its commitment and immediately cancel the wrong decision," vice foreign minister Zhang Zhijun told Locke, according to the official Xinhua news agency.

China snapped military exchanges with the United States in January 2010 after President Barack Obama's administration approved an earlier $6.4 billion package that included helicopters and Patriot missiles, although cooperation resumed within months.

The Obama administration had kept quiet for months over Taiwan's request, first lodged in 2007, to buy 66 F-16 C/D fighters -- which have better radar and more powerful weapons systems than its F-16 A/Bs -- in response to China's growing military muscle.

The decision to upgrade the existing fleet was seen by some observers in Taiwan as a consolation prize. Obama also came under fire from members of the rival Republican Party who said that jet sales would have created badly needed US jobs.

Republican presidential candidate Mitt Romney said Obama "caved in to the unreasonable demands of China at the cost of well-paying American jobs."

Representative Ileana Ros-Lehtinen, the head of the House Foreign Affairs Committee, called the upgrade "a modest step in the right direction" but "woefully insufficient."

Senator John Cornyn, whose state of Texas stands to benefit from arms sales, accused Obama of failing to meet obligations under the 1979 Taiwan Relations Act, which requires Washington to ensure Taiwan's self-defense.

The arms decision "bestows upon Communist China a newfound sway over American national security, and this capitulation should be met with concern by US allies everywhere," said Cornyn, who has introduced legislation that aims to force the sale of 66 new F-16s.

A senior administration official shot back at the criticism, saying that the more than $12 billion in arms approved for Taiwan under Obama contrasted favorably with the record of Republican President George W. Bush.

"Essentially, we have provided twice the amount in half the time. And that is a substantial commitment and I don't think it's fair to turn this into a partisan issue," the official said on condition of anonymity.

The official said that the United States was not ruling out future sales of new F-16 C/D jets but said that the retrofit was the best option in the near term.

"It is our belief that we are going to be able to get greater capabilities more rapidly and a large number of airplanes into the field in a more decisive way," he said.

Richard Aboulafia, an aviation expert and vice president of the Teal Group Corp., said that Taiwan's F-16 A/Bs were already unusually modern and included many of the technical features of C/Ds.

But he said that the upgrade decision meant that Taiwan would not increase its fleet size at a time that China is on the rise.

"This is a numbers problem," he said.

The package, which will take place over 12 years, also includes training for Taiwan's pilots at Luke Air Force Base in Arizona.

China's defeated nationalists fled to Taiwan after losing the civil war in

1949 and the island has since transformed into a vibrant democracy. US officials reiterated that Washington still only recognizes Beijing.

Ties between China and Taiwan have improved markedly since Beijing-friendly President Ma Ying-jeou took office in 2008 and ramped up trade and other links.

But China has refused to renounce the use of force against the self-governing island and Ma has publicly sought new F-16s.