By Rosalind Russell and Alison Williams | Reuters 

ISLAMABAD (Reuters) - Pakistan gave China access to the previously unknown U.S. "stealth" helicopter that crashed during the commando raid that killed Osama bin Laden in May despite explicit requests from the CIA not to, the Financial Times reported on Sunday.

The disclosure, if confirmed, is likely to further shake the U.S.-Pakistan relationship, which has been improving slightly after hitting its lowest point in decades following the killing of bin Laden.

During the raid, one of two modified Blackhawk helicopters, believed to employ unknown stealth capability, malfunctioned and crashed, forcing the commandos to abandon it.

"The U.S. now has information that Pakistan, particularly the ISI, gave access to the Chinese military to the downed helicopter in Abbottabad," the paper quoted a person "in intelligence circles" as saying on its website.

It said Pakistan, which enjoys a close relationship with China, allowed Chinese intelligence officials to take pictures of the crashed aircraft as well as take samples of its special "skin" that allowed the American raid to evade Pakistani radar.

One U.S. official, speaking on condition of anonymity, told Reuters there was reason to believe Pakistan had allowed the Chinese to inspect the aircraft. But the official could not confirm it happened with certainty.

No one from the Pakistani army was available for comment, but the Inter-Services Intelligence Directorate (ISI), Pakistan's top spy agency, denied the report. The paper said Pakistan's top general, chief of army staff Ashfaq Kayani, denied that China had been given access.

The surviving tail section, photos of which were widely distributed on the Internet, was returned to the United States following a trip by U.S. Senator John Kerry in May, a spokesman for the U.S. embassy told Reuters.

Shortly after the raid, Pakistan hinted that it might give China access to the helicopter, given its fury over the raid, which it considers a grievous violation of its sovereignty.

"We had explicitly asked the Pakistanis in the immediate aftermath of the raid not to let anyone have access to the damaged remains of the helicopter," the Financial Times quoted the source as saying.

In an incident such as the helicopter crash, it is standard American procedure to destroy sophisticated technology such as encrypted communications and navigation computers.

DISPLEASURE

Pakistan is a strategic ally to the United States but the relationship has been on a downward spiral since the killing of the al Qaeda leader in the raid by U.S. forces.

Islamabad was not informed in advance and responded by cutting back on U.S. trainers in the country and placing limits on CIA activities there.

The fact that the al Qaeda chief lived for years near the Pakistani army's main academy in the northwestern garrison town of Abbottabad reinforced suspicions in Washington about Islamabad's reliability in the war against militant Islamists.

There are also growing frustrations with Pakistan over its reluctance to mount offensives against militant factions in the northwest who are fighting U.S.-led foreign forces across the border in Afghanistan.

In a show of displeasure over Pakistan's cutback in U.S. trainers, its limits on visas for U.S. personnel and other bilateral irritants, the United States has suspended about a third of its $2.7 billion annual defense aid to Pakistan.

Despite this, both sides have tried to prevent a breakdown of relations.

The head of Pakistan's powerful Inter-Services Intelligence (ISI), Lieutenant-General Ahmad Shuja Pasha, visited the United States last month for talks with U.S. government and intelligence officials, which both sides said went well.

Despite the billions in aid, Pakistan still considers China a more reliable ally than the United States. China is a major investor in predominantly Muslim Pakistan in areas such as telecommunications, ports and infrastructure. The countries are linked by a Chinese-built road pushed through Pakistan's northern mountains.

Trade with Pakistan is worth almost $9 billion a year for Pakistan, and China is its top arms supplier.

In the wake of attacks that left 11 people dead in the China's western region of Xinjiang in late July, Pakistan dispatched the ISI's Pasha to Beijing.

(Writing and additional reporting by Chris Allbritton in Islamabad, Mark Hosenball and Phil Stewart in Washington; Editing by Rosalind Russell and Alison Williams)