SIMI VALLEY, Calif. — Defense Secretary Lloyd J. Austin III said on Saturday that the Biden administration would push both allies in the Pacific and U.S. businesses to step up efforts to counter the increasingly urgent threat of China, and that the Pentagon was prepared to help Ukraine better defend itself against Russia as tensions flare between the two countries.

“America is a Pacific power,” Mr. Austin declared during a speech at the annual Reagan National Defense Forum, the first in-person meeting of defense officials and experts since the start of the coronavirus pandemic. “We’re not asking countries to choose between the United States and China. Instead, we’re working to advance an international system that is free and stable and open.”

In one of his most prominent speeches since the United States pulled troops out of Afghanistan, bringing a chaotic end to America’s longest war, Mr. Austin made little mention of the past two decades of U.S. military efforts in the Middle East, turning the focus instead almost exclusively on Beijing, whose nuclear, cyber and economic gains have increasingly rattled American officials over three administrations.

He offered the broad outlines of his strategy for dealing with China, something he called “integrated deterrence,” which relies on both strengthening work with allies and partners in the region and pushing the U.S. technology industry to keep ahead of Chinese innovations.

The Biden administration has carefully tried to both insist that the United States is not in conflict with China, and acknowledge that the competition between the two countries has intensified. President Biden has declined to lift tariffs initiated by the Trump administration and has continued to push China to uphold commitments it had agreed to as part of a trade deal signed during the final days of the Trump administration.

In panel after panel on Saturday at the Ronald Reagan Presidential Library and Museum, military officials and defense and national security experts rang alarm bells about China, including its increasing military power and its attacks on American satellite, as well as economic concerns such as the lack of skilled American workers to keep up with Chinese production and dominance, especially in semiconductors.

“There is a real possibility if we ever got into a conflict, you could see attacks on our power grid or transportation sector,” warned Christine Wormuth, the secretary of the Army.

At the same time, speakers also expressed concern that President Vladimir V. Putin of Russia might exploit the United States’ intense focus on China to quietly pursue his own ambitions in his region.

A new report from American intelligence agencies shows Russia has drawn up plans to possibly invade Ukraine with as many as 175,000 forces as soon as early next year. Intelligence officials do not believe Mr. Putin has made a decision on whether to attack Ukraine, but the threat of a renewed invasion has become more acute.

Mr. Austin noted that Russia had invaded Ukraine before, a reference to 2014 when Moscow annexed Crimea and its military directed pro-Russian separatists in eastern Ukraine.

The current troop buildup in the border region, as well as Russian disinformation activity and cyberoperations, are all concerning to the United States, Mr. Austin said.

“We remain focused on this,” Mr. Austin said. “We are certainly committed to helping Ukraine defend its sovereign territory.”

The Biden administration has been trying to use concerns over both Russia’s and China’s intentions to bolster support among allies.

In his remarks, Mr. Austin stressed that the United States did not intend to build a new NATO for Asia. Rather, Washington is trying to better coordinate countries to block efforts by China to dominate the region, he said, pointing to the recent deal to help Australia deploy nuclear-powered submarine as an example of strengthening alliances.

“They are moving on their goals,” David H. Berger, commandant of the U.S. Marine Corps, said of China. “We have to operate in a different way.”

The so-called pacing challenge of China — a phrase often used in American policy circles to denote that nation’s steadily increasing military threat — has become of largely bipartisan interest in Congress.

In June, the Senate overwhelmingly passed ​​a bill to spend nearly a quarter-trillion dollars to ignite scientific innovation to better compete with China, a level of investment that proponents say will be comparable to Cold War-era spending if the House follows suit.

There was roughly an equal number of congressional Democrats and Republicans at the forum and all expressed similar concerns.

“We need to be there,” said Senator Tammy Duckworth, Democrat of Illinois, who stressed the need to invest in partners in the region. Some Republicans, however, were critical of Mr. Biden’s policies thus far toward China, particularly around what they view as a lack of punitive measures over the country’s role in the pandemic. “President Biden has to be much more aggressive,” Senator Joni Ernst, Republican of Iowa, said.

Although Mr. Austin said U.S. tech firms needed to help the country keep ahead of Chinese know-how, he acknowledged that the Pentagon risked falling behind China in various areas if it did not find ways to work better with Silicon Valley.

“The barriers to entry for working in national security are often just too steep,” Mr. Austin said.

Amid all of the talk of future threats and partnerships within the crowd of senior Pentagon officials, policy experts and business leaders, there was little reflection on the past two decades of war in Iraq and Afghanistan.

When pressed after his speech about his regrets over the Afghanistan withdrawal, Mr. Austin was silent for several seconds, before saying that he regretted the loss of lives of Marines and of civilians killed in an errant drone strike. “I want to make sure that we don’t lost sight of the fact that our American forces in 17 days evacuated 124,000 people from Afghanistan,” he added.