The United States, France and Britain moved swiftly yesterday to freeze tens of billions of dollars worth of Kuwaiti investments and properties to prevent Iraq from using the spoils from its invasion of the Persian Gulf sheikhdom to relieve its own desperate economic condition.

The Bush administration also froze Iraqi assets and banned imports of Iraqi oil and other products, moves that will probably make it harder, though not impossible, for Iraq to carry on foreign trade and rebuild its war-torn economy.

Customs officials yesterday impounded a load of Iraqi crude oil from a tanker bound for a refinery in Houston, and there were unconfirmed reports that another shipment may also have been impounded.

Iraq retaliated by freezing payments on the U.S. portion of its estimated $40 billion worth of foreign debts, including payments on $2.24 billion owed to the U.S. government.

The freeze on Kuwait's investments represents the most far-reaching imposition of economic sanctions since World War II. The action takes aim at Kuwaiti government investments that amount to as much as $100 billion and that include, among other things, massive holdings of U.S. government securities, chains of gasoline service stations across Europe, a giant American oil services company, an interest in the London dockyards development and stakes in Mercedes Benz, Marine Midland Bank, British Petroleum and the London-based Savoy Hotel chain.

"We're dealing with a far more significant financial power than ever has been blocked before," said Robert Hormats, an investment banker and former State Department official. "Kuwait is a very diversified and very sophisticated investor. Making the freeze work effectively will be far more complicated than earlier sanctions."

To keep Kuwait's foreign holdings out of the hands of Iraq or its Kuwaiti surrogates would require the cooperation of other U.S. allies, especially Switzerland, West Germany, Japan, Austria and South Korea, Hormats said.

Yesterday Italy said it was considering taking action and the Swiss government told Swiss banks to increase their vigilance and to reject any withdrawals of Kuwaiti assets that seem suspicious. It also urged Swiss banks to examine the identities of account holders and those seeking to withdraw funds.

The fate of Kuwaiti assets is crucial because financial issues are seen as central to the clash between the two countries.

Iraq's financial needs are as important as its geopolitical ambitions in the region, many analysts said. "This is an oil and bank robbery combined," said Hormats. "Iraq's coffers are pretty well drained. That is certainly part of the motivation."

Iraq has sunk deeply in debt as a result of ambitious development plans and its lengthy war with Iran.

By contrast to Iraq, Kuwait is virtually swimming in oil and money. With a small population and giant oil reserves, Kuwait has run huge financial surpluses for two decades, even after building extensive development projects at home. Much of the money was put in a fund, known as the reserve fund, for future generations.

Some economists and bankers estimate that the revenue from the sheikhdom's foreign investments exceeds revenue from oil exports.

That money and the diplomatic agility of the Kuwaiti royal family were the only weapons at the government's disposal.

"Historically, Kuwait expected that money could fix everything and therefore it didn't expect military action," said Christopher Keane, general manager of the London branch of the Union Bank of Kuwait, which is partly owned by the Kuwaiti government.

While cautious in its investment strategy, Kuwait also has been creative, branching into "downstream" oil businesses, extending its activities to refining and marketing of oil and oil products. In the United States, it bought Santa Fe International Inc., one of the world's biggest suppliers of oil services. In Europe, it has acquired four refineries and more than 5,000 stations that operate under the name Q-8.

It has spread its investments among a wide variety of managers as well as countries, placing between a third and half of its investments in this country and about $28 billion in Europe, according to estimates by bankers.

The Treasury Department said that Kuwait has $4.2 billion in direct investment holdings in the United States. Santa Fe represents the biggest chunk of that, but Kuwait also owns some New York real estate and has interests in the Saks Fifth Avenue department store chain through a Bahrain-based investment group.

Sources said that one of Kuwait's biggest advisers is the New York investment bank Morgan Stanley & Co., which manages about $3 billion worth of Kuwaiti assets. Citibank also manages a chunk of Kuwaiti assets, investment banking sources added.

Despite the far-reaching network of Kuwait's investments, bankers said they expected little financial market disruption as a direct result of the freeze, though fears arising from political instability in the oil-rich Persian Gulf caused widespread nervousness.

"The investment funds can continue to be managed in the same way. No disruption is necessary," said David F. Lomax, group economic adviser for London-based National Westminster Bank.

"The financial markets will cope with this kind of thing. The assets are not going to go anywhere. They are all going to be gridlocked for the foreseeable future," said Keane.

As a result, many bankers and lawyers were skeptical of Iraq's ability to take advantage of Kuwait's offshore assets. The invasion "is a pretty good indication that Iraq's financial situation is desperate, but it is not immediately obvious how they come out better," said Keane. "The question is whether they can sell oil or whether people will be able to pay for it."

Indeed, the sanctions could compound Iraq's trade and payments problems. The United States and Europe are the biggest suppliers of grain to Iraq, much of that financed through the Export-Import Bank and by Commodity Corp. credits. While Iraq still should be able to sell its oil through middlemen, it might pay an additional price to circumvent sanctions.

Officials at several oil companies said they were hoping to convince the U.S. government to let Iraqi oil already on its way to the United States come here as planned, but they also said they were prepared to divert the shipments elsewhere as needed.

One of the loads impounded yesterday was bound for a refinery operated by Lyondell Petrochemical Co. of Houston. A spokeswoman for the company was unable to say how much oil was affected, but she said Lyondell would be forced to reduce its refinery runs by about 25 percent. "This was oil we thought would be available to us," said the spokeswoman, Jackie Wilson.

"Our guess is there's probably 20 to 25 million barrels of crude oil that has been shipped from Iraq and Kuwait that hasn't yet reached the United States," said Pete Leutwiler, vice president of supply and logistics for Citgo Oil Co.

"We have seen in other sanction situations, that countries manage to survive, but it could put a major crimp in their style," said Jodi Nelson, a lawyer with Rogers and Wells, a firm that represented major U.S. banks that were involved in the freezes of Iranian and Libyan assets.

Staff writers John Burgess and Mark Potts contributed to this report.