The embattled Yukos Oil Co. warned Thursday that it was going to run out of cash within three weeks because of the government freeze on its assets and bank accounts, jeopardizing the operations of Russia's largest oil producer and potentially disrupting exports to world markets.

Unless the government lifts its legal restrictions, Yukos said it would face insolvency by mid-August and would no longer be able to meet its business obligations. Moreover, if the government follows through on its plan to sell off Yukos's primary production unit to satisfy a back tax claim, the company said it would be forced into bankruptcy.

"The next three weeks are going to be very critical to the company's future," Steven M. Theede, Yukos's American chief executive officer, said at a news conference. He added: "To the extent that the bills can't be paid . . . it's very difficult to continue as a going concern."

While previous Yukos warnings of bankruptcy and shutdown have been met with skepticism in Russia, investors and analysts now say that what some have called the doomsday scenario seems increasingly credible. The government's announcement Tuesday that it would seize and sell the company's core Yuganskneftegaz subsidiary turned even some of the most bullish investors pessimistic about the future of both the company and the country.

Yukos stock fell another 16 percent Thursday to $5.05 a share -- its lowest level since Christmas 2001 -- for a cumulative 36 percent loss since Tuesday's announcement. All told, Yukos has lost about 66 percent of its market value, or about $30 billion, since its legal battle with the state escalated in April.

In addition to idling as many as 100,000 workers, a Yukos shutdown in mid-August could drive up international oil prices if it reduces overseas sales by Russia, which now rivals Saudi Arabia as the world's largest oil producer and has become the world's second-largest exporter. Yukos pumps 1.7 million barrels of crude oil per day, roughly the equivalent of Libya, and exports more than any Russian company.

In testimony before the U.S. Senate this week, Alan Greenspan, chairman of the Federal Reserve, identified the problems at Yukos as a factor pushing up international oil prices. The price of U.S. light crude, a widely cited benchmark of market conditions, inched up another 78 cents to close at $41.36 a barrel within hours of the Yukos statements Thursday.

Yukos said it had paid for pipeline access through August but might have to curtail exports sooner if it cannot operate its wells. Transneft, the state pipeline company, has said it would find other crude oil to replace Yukos supplies if necessary, but Theede predicted "there would certainly be some short-term impact on exports."

The company has reached the brink of collapse after being hit with nearly $7 billion in back tax bills as part of a politically charged campaign against its imprisoned chief shareholder, Mikhail Khodorkovsky, a rival of President Vladimir Putin. The bills resulted from a government decision to revoke tax shelters that it had previously approved.

By freezing its assets, as well as bank accounts that control about half of the company's revenue, the state has made it more difficult for Yukos to pay. Another $200 million in cash was frozen Wednesday, Yukos said, and lenders won't give the company more money.

Although it missed a court-ordered deadline to pay the first $3.4 billion in tax claims, Yukos has begun paying portions of the bill. So far, it said, it has paid $300 million. It said it expects to pay another $400 million by the end of the month, in addition to waiving its right to $550 million in refunds of value-added taxes.

Theede said the company had sent 11 letters to the government seeking to negotiate a payment schedule or some other deal, all of which remain unanswered. "We're not sure exactly what they want," he said. "We'll try and continue to come up with other alternatives that might be interesting to them."

Yukos CEO Steven M. Theede, right, said at a news conference that the government had not responded to any of its 11 settlement proposals.