With water levels in the Colorado River near their lowest point ever, Arizona and Nevada on Tuesday faced new restrictions on the amount of water they can pump out of the river, the most important in the Southwest.

And the threat of more cuts looms. This week, those two states along with five others failed to meet a deadline for agreement on much steeper cuts in water use, raising the prospect that the federal government will step in and mandate further reductions.

The Colorado, which supplies water to 40 million people in the United States and Mexico and supports billions of dollars of agricultural production across the region, is in the throes of two decades of drought made worse by climate change. At the river’s two immense reservoirs, Lake Mead and Lake Powell, water levels are at just 28 percent of total capacity because of the river’s diminished flow and increased demand.

As the water crisis intensified last year, the federal government for the first time imposed restrictions on the amount of water that could be taken. Tuesday’s cuts are smaller than those put in place a year ago and also affect Mexico. And they would be dwarfed by much larger cuts that the federal government in June asked the states to negotiate among themselves and threatened to impose if the states do not reach agreement.

Officials have said the large cuts — involving 20 to 40 times as much water as Tuesday’s — would be needed next year to protect the reservoirs and the dams that created them and to stabilize the water supply. Engineers point out that if water levels continue falling, the Glen Canyon Dam at Lake Powell near the Arizona-Utah border will eventually no longer be able to generate hydropower.

“States collectively have not identified and adopted actions of significant magnitude that would stabilize the system,” said Camille Camlimlim Touton, commissioner of the Bureau of Reclamation, the federal agency with the authority to impose cuts, speaking at a news conference.

The negotiations among the seven states have proceeded slowly, marked by the kind of finger-pointing that has complicated talks over Western water rights for much of the past century. John Entsminger, general manager of the Southern Nevada Water Authority, in a letter to Ms. Touton and other officials this week, said that despite the obvious urgency of the situation, the negotiations had produced “exactly nothing in terms of meaningful collective action to help forestall the looming crisis.”

In an interview following Tuesday’s announcement, Mr. Entsminger said that the Bureau of Reclamation appeared to be encouraging states to keep negotiating. “But they’re also beginning to take the steps to put the tools together that they say they need to impose those cuts,” he said.

Jennifer Pitt, Colorado River program director at the National Audubon Society, said there had been intense pressure on all the stakeholders to come up with a plan for the steep cuts. “The water’s just not there,” she said. “That’s the stone-cold reality, and no amount of politicking can change that.”

Tuesday’s announcement of cuts was triggered by the fact that Lake Mead, the immense reservoir behind Hoover Dam at the Arizona-Nevada border, is now about 175 feet lower than it was in 2000, when the current Southwestern drought began.

Combined with the larger reductions imposed last year, the new cuts mean that Arizona will have access to 21 percent less water annually, compared to its historic allocation. Nevada’s total reductions are now about 8 percent of its allocation. Mexico’s cuts total 7 percent of its allotted supply.

Mr. Entsminger said that conservation measures already undertaken in Las Vegas and surrounding cities, including a recent ban on water-hogging turf that serves no purpose other than decoration, would enable residents to weather the new cuts.

In Arizona, the cuts so far have largely affected farmers in the central part of the state, who have had to fallow fields or shift to less water-intensive crops to get by. Some farmers have left the business entirely.

When it comes to the sharp reductions called for by Ms. Touton, farmers are expected to be most affected as well. Agriculture uses about three-quarters of the Colorado supply.

During the discussions over the cuts, some agricultural groups floated the idea of farmers being compensated for taking some of their land out of production to conserve water. There is money in the just-signed Inflation Reduction Act that potentially could be used for such a program.

The call for steeper cuts reflects deep worries in the Bureau of Reclamation about the ability to maintain safe operations, and to generate hydropower, as runoff into the Colorado continues to decline.

The loss of a steady supply of electricity from the Glen Canyon Dam could make the Western power grid less stable. And the ability of the dam to pass any water at all downstream might be at risk.

Mr. Entsminger said that despite the dire situation, he thought an agreement between states to address the crisis remained unlikely. “I feel like we haven’t reached the point where every water user on the river accepts that everybody has to be a part of this solution,” he said.

Ms. Pitt said that if the states came back to the bureau in a month or two with a plan for cuts, “the federal government, I’m sure, would be happy to take that because no elected leader wants to be responsible for the decision about who’s not going to get water.”