NEW YORK —Even as Greece desperately tries to avoid defaulting on its debt, US companies are preparing for what was once unthinkable: that Greece will soon be forced to leave the eurozone.

Bank of America Merrill Lynch has looked into filling trucks with cash and sending them over the Greek border so clients could pay local employees and suppliers in the event money is unavailable. Ford has configured its computer systems so they will be able to immediately handle a new Greek currency.

No one knows just how broad the shock waves from a Greek exit would be, but big US banks and consulting firms have been doing a brisk business advising corporate clients on how to prepare for a splintering of the eurozone.

That is a striking contrast to the assurances from European politicians that the crisis is manageable and the currency union can be held together. On Thursday, the European Central Bank will consider measures that would ease the pressure on Europe’s­ cash-starved countries.

JPMorgan Chase, though, is taking no chances. It has created new accounts for a handful of US giants that are reserved for a new drachma in Greece, or whatever currency might succeed the euro.

In a survey, the firm found that 80 percent of clients polled expected Greece to leave the eurozone; a fifth of those expected more countries to follow.

‘‘Fifteen months ago when we started looking at this, we said it was unthinkable,’’ said Heiner Leisten, a partner with Boston Consulting Group in Cologne, Germany. ‘‘It’s not impossible or unthinkable now.’’

Leisten’s firm, as well as PricewaterhouseCoopers, has considered the timing of a Greek withdrawal — for example, the news might hit on a Friday night, when global markets are closed. A bank holiday could quickly follow, with the stock market and local financial institutions shutting down, while new capital controls make it hard to move money in and out of the country.

‘‘We’ve had conversations with several dozen companies and we’re doing work for a number of these,’’ said Peter Frank, who advises corporate treasurers as a principal at Pricewaterhouse.

He added: ‘‘Companies are asking some very granular questions, like ‘If a news release comes out on a Friday night announcing that Greece has pulled out of the euro, what do we do?’ In some cases, companies have contingency plans in place, such as having someone take a train to Athens with 50,000 euros to pay employees.’’

The preparations signal a stark switch, said Carole Berndt, head of global transaction services in Europe, the Middle East and Africa for Bank of America Merrill Lynch.

‘‘When we started giving advice, they came for the free sandwiches and chocolate cookies,’’ she said jokingly. ‘‘Now that has changed, and contingency planning is focused on three primary scenarios: a single-country exit, a multicountry exit, and a breakup of the eurozone in its entirety.’’

Banks and consulting firms are reluctant to name clients, and many big companies declined to discuss their contingency plans, fearing it could anger customers in Europe if it became known they were contemplating the euro’s demise.

Central banks, as well as Germany’s finance ministry, have also been considering the implications of a Greek exit but have been even more secretive about specific plans.

But some corporations are beginning to acknowledge they are ready if Greece or even other countries leave the eurozone, making sure that systems can handle a quick transition to a new currency.

In Europe, the holding company for Iberia Airlines and British Airways has acknowledged it is preparing plans in the event of a euro exit by Spain.

‘‘We’ve looked at many scenarios, including where one or more countries decides to redenominate,’’ said Roger Griffith, who oversees global settlement and customer risk for MasterCard. ‘‘We have defined operating steps and communications steps to take.’’

He added, ‘‘Practically, we could make a change in a day or two and be prepared in terms of our systems.’’

In a prepared statement, Visa­ said that it, too, would be able to make ‘‘a swift transition to a new currency with the minimum possible disruption to consumers and retailers.’’

US companies have actually been more aggressive about seeking out advice than their European counterparts, according to John Gibbons, the head of treasury services in Europe for JPMorgan Chase.

He said that a handful of the largest US companies had requested the special accounts configured for a currency that did not yet exist.

‘‘We’re planning against the extreme,’’ he said. ‘‘You don’t lose anything by doing it.’’