WASHINGTON, May 21 (Reuters) - U.S. Defense Secretary Lloyd Austin has so far been unable to speak with China's top general despite multiple attempts to set up talks, U.S. defense officials said on Friday.

Relations between China and the United States have grown increasingly tense, with the world's two largest economies clashing over everything from Taiwan and China's human rights record to its military activity in the South China Sea.

Despite the tensions and heated rhetoric, U.S. military officials have long sought to have open lines of communication with their Chinese counterparts to be able to mitigate potential flare-ups or deal with any accidents.

"The military relationship is strained, no question about that. It’s hard to know how much this is reflective of that strain as much as it is just Chinese intransigence," a U.S. defense official said.

"But we certainly want to have a dialogue. We just want to make sure we have a dialogue at the proper level," the official, speaking on the condition of anonymity, added.

China's embassy in Washington could not immediately be reached for comment.

A second U.S. official said there was a debate in President Joe Biden's administration about whether Austin should speak with vice chairman of China’s Central Military Commission, Xu Qiliang, or Chinese Defense Minister Wei Fenghe.

Xu is seen as having more power and influence with Chinese President Xi Jinping.

Austin would have had an opportunity to meet with Wei in Singapore later this month during a conference attended by defense ministers from the region, but the event was canceled because of the COVID-19 pandemic.

The unsuccessful attempts by the Pentagon to reach out to the Chinese were first reported by Financial Times.

Late last year senior Chinese and U.S. defense officials held talks on crisis communication.

While there have not been high level military talks since Biden took office in January, senior diplomats from the two countries met in Alaska in March. Those talks bristled with rancor and yielded no diplomatic breakthroughs.

ABUJA, May 21 (Reuters) - Nigeria's army chief, Lieutenant General Ibrahim Attahiru, died in a plane crash on Friday on an official visit to the northern state of Kaduna, which has had security challenges in recent months, the presidency said.

The air force said in a statement that its plane crashed near Kaduna airport and that it was investigating the cause.

The presidency said the army general and other military officers died in the crash on Friday.

President Muhammadu Buhari, in a presidency statement, described the crash as a "mortal blow ... at a time our armed forces are poised to end the security challenges facing the country."

The crash occurred three months after a small Nigerian air force passenger plane went down just outside Abuja airport following alleged engine failure, killing all seven people on board. read more

Nigeria, Africa's most populous nation, has had poor air safety record in the past, although it has improved in recent years.

Buhari appointed Attahiru alongside other military chiefs in January after years of mounting criticism over spreading violence by Islamist insurgents and armed gangs.

Boko Haram and its offshoot, Islamic State West Africa Province, have waged a decade-long insurgency estimated to have displaced about 2 million people and killed more than 30,000. They want to create states based on their extreme interpretation of Islamic sharia law.