TOKYO » Seeking to soothe an anxious ally, the U.S. voiced solidarity with Japan on Tuesday (Monday in Hawaii) against China’s claim to airspace over disputed islands, vowing not to tolerate the aggressive move as U.S. Vice President Joe Biden prepared to deliver that message personally to Beijing.

Standing shoulder to shoulder in Tokyo with Japanese Prime Minister Shinzo Abe, Biden said the U.S. is "deeply concerned" about China’s attempt to unilaterally change the status quo in the East China Sea. He said he would raise the issue "with great specificity" when he meets Thursday with Chinese President Xi Jinping.

"This action has raised regional tensions and increased the risk of accidents and miscalculation," Biden said.

China’s recent move to assert authority over the airspace cast a shadow over Biden’s first day in Asia, where the vice president also sipped coffee with Japanese lawmakers and toured a tech company’s headquarters overlooking Tokyo’s sprawling skyline.

The U.S., Japan and other American allies have refused to recognize China’s new air defense zone, widely seen as an attempt to solidify China’s claim to the islands as part of a broader effort to launch a show of dominance in the region.

But Washington is also wary of creating a new fault line in its relationship with Beijing that could complicate its pursuit of a new era of economic cooperation, forcing the Obama administration to perform a delicate diplomatic dance as it responds to a simmering conflict that has put the entire region on edge.

"The United States has an interest in the lowering of tensions in this part of the region, as I believe all the countries in Northeast Asia share that interest with us," Biden said after meeting here with Abe at the Kantei, the prime minister’s official residence.

As Biden headed to the region Monday for a weeklong trip to Japan, China and South Korea, Tokyo was pressing the U.S. to more actively take its side in the escalating dispute.

One issue involved U.S. guidance to American commercial airlines about complying with airspace restrictions, which Japan perceived as potential acquiescence to China. Reluctant to cede any ground, Tokyo has been urging Japanese commercial flights not to notify China before flying through the zone.

The zone covers more than 600 miles from north to south, above international waters separating China, South Korea, Japan and Taiwan. China says all aircraft entering the zone must notify Chinese authorities beforehand or face unspecified defensive measures.

Invoking the decades-old defense treaty between the U.S. and Japan, Abe said he and Biden confirmed that neither country should tolerate China’s move, adding that both countries’ air forces would continue to fly through without filing flight plans with China.

But Abe appeared to smooth over any conflict between the U.S. and Japan over commercial flights as he and Biden spoke to reporters after their meeting.

"We further agreed we will not condone any actions that could threaten safety of civilian aircraft," Abe said, without elaborating.

Earlier Tuesday, senior Obama administration officials said the U.S. never told American commercial carriers to comply specifically with China’s demands. Rather, the Federal Aviation Administration merely reaffirmed existing policy that pilots should comply with such instructions anywhere in the world, said the officials, who weren’t authorized to comment by name and demanded anonymity.

Japan, which claims the islands as its own, is concerned that China’s move may presage further steps to assert control in the region. On Monday, China’s ambassador to the Philippines claimed China has a sovereign right to establish a similar zone over the South China Sea, where China and the Philippines are locked in another long-running territorial dispute.

The feud promises to trail Biden throughout his weeklong trip to Asia — a tour intended to affirm Washington’s continued interest in upping its presence in the region, in part to counter China’s growing influence.

After a working dinner with Abe on Tuesday night, Biden will fly to Beijing on Wednesday to meet with Chinese President Xi Jinping, where officials said Biden would raise U.S. concerns over the zone directly. Biden will then travel Thursday to South Korea — another U.S. ally at odds with China over the air defense zone.

The U.S. doesn’t take a position on the islands’ sovereignty but acknowledges that Japan administers them, meaning U.S. treaty obligations to defend Japan could come into play.

After a morning meeting with Japanese lawmakers and Deputy Prime Minister Taro Aso, Biden made his way to Shibuya, a bustling Tokyo district and fashion center, where he toured a technology company founded by a female entrepreneur to highlight the role of women in Japan’s economy. U.S. Ambassador to Japan Caroline Kennedy joined Biden as he mingled with young employees in a chic, 24th floor lunchroom overlooking Tokyo’s sprawling skyline.

At a round table later with business executives, Biden said he’d heard some say women are good in the workforce because they are kinder and gentler.

"I’ve never found that to be the case," Biden said to laughter. "They’re as tough, they’re as strong, they’re as everything as a man is — and vice-versa."