A Chinese surveillance balloon is collecting intelligence over the continental United States "right now," senior U.S. officials disclosed Thursday, acknowledging that the Pentagon has been monitoring the craft for several days and briefly considered shooting it down before concluding that doing so posed a safety risk.

The balloon is traveling at an altitude "well above commercial air traffic and does not present a military or physical threat to people on the ground," Brig. Gen. Patrick Ryder, a Pentagon spokesman, told reporters in a hastily arranged news conference where he addressed the ongoing situation. The North American Aerospace Defense Command, or NORAD, continues to track the balloon's course, but officials would not specify its present whereabouts.

"Once the balloon was detected," Ryder added, "the U.S. government acted immediately to prevent against the collection of sensitive information." Without elaborating, he said that similar activity had been observed before - spanning a period of "several years."

A U.S. intelligence official, speaking on the condition of anonymity because of the sensitivity of the issue, said that similar balloons have been spotted previously over Hawaii and Guam. They have guidance systems on board, the official said.

The striking development comes at a time of peak tension between the world powers, and just hours ahead of Secretary of State Antony Blinken's expected departure for Beijing, where he is to hold a series of long-scheduled meetings with senior Chinese officials. The high-stakes visit, Blinken's first to the country as the nation's top diplomat, is aimed at stabilizing U.S.-China relationship - a goal that could become more difficult following the espionage aircraft's appearance in U.S. airspace.

The State Department did not immediately respond to a request for comment, nor did the Chinese Embassy in Washington.

Thursday's disclosure elicited a furious response from lawmakers in both political parties. Reps. Mike Gallagher (R.-Wis.) and Raja Krishnamoorthi (D.-Ill.), the leaders of a House select committee on China, issued a joint statement saying "the Chinese Communist Party should not have on-demand access to American airspace."

"Not only is this a violation of American sovereignty, coming only days before Secretary Blinken's trip to the [People's Republic of China], but it also makes clear that the [Chinese Communist Party's] recent diplomatic overtures do not represent a substantive change in policy," they said, adding the incident demonstrates that the threat posed by China "is not confined to distant shores - it is here at home and we must act to counter this threat."

The incident, first reported Wednesday by NBC News, prompted a series of unusual events as the balloon was observed Wednesday over sparsely populated Montana, officials said. The state is home to numerous U.S. nuclear missile silos.

President Biden, upon being briefed about the development, requested military options, officials said. Defense Secretary Lloyd Austin, traveling in the Philippines, convened a meeting with senior advisers to assess how the United States might respond, and they at least briefly discussed shooting down the balloon, one official said in a conference call with reporters. This person, like others, spoke on the condition of anonymity under ground rules set by the Pentagon.

Senior military officers, including Gen. Mark A. Milley, chairman of the Joint Chiefs of Staff, advised against such a move, citing concerns that falling debris could put people and property at risk, the senior official said.

The balloon's flight path takes it over "a number of sensitive sites," this official said, but it appears it does not have the ability to collect information that is "over and above" other tools at China's disposal, such as low-orbit satellites. Nevertheless, the Pentagon is taking undisclosed "mitigation steps" to prevent Beijing from gathering additional intelligence.

"We know exactly where this balloon is, exactly what it is passing over, and we are taking steps to be extra vigilant so that we can mitigate any foreign intelligence risk," the official said.

This official acknowledged that, as the Pentagon contemplated how to respond as the balloon was over Montana, civilian flights in the area were halted and U.S. military aircraft, including advanced F-22 fighter jets, were sent to investigate.

"We wanted to make sure we were coordinating with civil authorities to empty out the airspace around that potential area," the official said. "But even with those protective measures taken, it was the judgment of our military commanders that we didn't drive the risk down low enough. So we didn't take the shot."

Before reaching the U.S. mainland, the balloon initially soared above Alaska's Aleutian Islands and then over Canada, another U.S. official said. It was not clear from where the balloon was launched initially.

The U.S. government under successive presidential administrations has identified China as Washington's top security concern. Washington has sought to expand partnerships in the Asia-Pacific region in response to what officials contend is increasingly aggressive behavior by Beijing. The Chinese government maintains it is the United States that has taken unwelcome and provocative actions, namely through supplying arms to Taiwan. U.S. officials are worried that China intends to invade the self-governing island, which Beijing claims as its own, within the next few years.

On Thursday, with Austin meeting with counterparts in the Philippines, the two nations announced a new agreement in which U.S. forces will receive access to four additional bases in the region, solidifying a months-long effort to expand the Pentagon's strategic footprint there in an effort to counter China. The plan adds to five other bases already in use for training, staging equipment and runway access under a 2014 defense cooperation agreement.

Japan also has announced new security plans with the United States, including the overhaul of a U.S. Marine Corps unit based in Okinawa to serve as an island-hopping force armed with ship-killing missiles.

China's brazen surveillance effort coincides with recent warnings from the U.S. Air Force over proposed Chinese land purchases in North Dakota about 12 miles from a military facility where drone test flights are conducted. The pending deal for a corn milling site has fueled concerns that the purchase is a cover for Chinese surveillance activities. A U.S. interagency committee decided last year it did not have jurisdiction to oppose the sale.

Air Force Assistant Secretary Andrew P. Hunter took an unambiguous view in a letter released earlier this week by North Dakota's senators.

"The proposed project presents a significant threat to national security," it says, "both near- and long-term risks of significant impacts to our operations in the area." The senators called for the project to be discontinued.

The Washington Post's John Hudson, Yasmeen Abutaleb and Shane Harris contributed to this report.