As if Iran's increasingly dangerous political problems were not enough, the country is deeply in debt, its banking system is on the verge of collapse and no money is coming in.

One result of all this is that many foreign companies -- including a number of American firms -- are losing hope of ever collecting substantial payments they are owed. And there is growing concern in financial circles here that Iran may be unable or unwilling to pay off other large bank debts.

A year ago, the country's credit rating ranked among the best in the would. Today there is no way to immediately service or repay the more than $2 billion owed to foreign financial institutions and companies, and virtually nobody will lend Iran another dime.

With employes of the central bank recurently on strikes or work slowdowns, telex communications largely cut off and the local banking systm functioning sporadically, if at all, Iranian debtors have been unable to make foreign exchange transfers to their foreign creditors.

Were these payments to resume immediately, though, they would drive Iran's foreign exchange reserves "dangerously low," bankers say. Only a few months ago, these reserves stood at $11.5 billion. Now they are estimated at about $7 billion to $8 billion, and resumption of foreign payments could cut them to $5 billion within two months.

"What appears to be looming is a further grinding into the ground of an already collapsed economy," one western banker said.

Trapped in the political and economic quagmire are seores of U.S. companies with contracts or industrial investments here.

Among the worst off are the defense contractors. They make up a major part of the American business community which until recently counted up to 45,000 American citizens and more than 500 U.S. firms in Iran.

Now more than two-thirds of those expatriates are estimated to have left the country, and some 10 to 15 percent of the U.S. companies here have closed up shop altogether.

Most of the American companies involved in Iran are contractors rather than investors. They largely work on a pay-as-you-go basis, submitting invoices to their customers as different stages of the job are completed.

"In theory, they should have no exposure," a banker said."But in practice, payments under the system have lagged many months behind. Most of the companies working on this basis have done several months work that they now cannot get paid for."

In addition, most contractors had to put up open-ended performance bonds on which the Iranian customer could draw if the firm failed to do the work according to its contract.

Although the strikes and political turmoil in many cases have brought work to a halt, a number of firms are at least maintaining a token presence to avoid losing all chance of collecting their performance bonds.

It is impossible to figure just how much money is owed to these American contractors, but banking sources estimated it could reach $100 million.

"American businessmen keep hoping things will improve. but with the political situation that seems to be developing, they are losing hope rapidly," a banker said. "Those who haven't been paid yet are probably not going to be, particularly the defense contractors."

The fear is that, with Iran's religious opposition pushing for an "Islamic republic," a new government would renege on military contracts signed by the regime of Shah Mohammed Reza Pahlavi, who flew into exile a week ago.

At stake are billions of dollars worth of contracts for construction of sophisticated military support facilities. Maintenance and operation of surveillance equipment, training programs and equipment deliveries.

The shah's opponents of both the left and right repeatedly have denounced hese contracts as examples of American "exploitation and imperialism." And Ayatollah Ruhollah Khomeini, the revered Shiite Moslem opposition leader who is expected to make a trimphant return from exile Friday, often has criticized the U.S. military role in Iran.

If Khomeini gets his way, many defense contracts could be canceled summarily, with heavy losses for both sides, economic analysts believe.

Among the U.S. companies facing contract cutbacks or payment suqeezes are Rockwell International, which reportedly is owed a substantial sum for work on the Ibex Project, a top-secret electronic surveillance and monitoring operation directed at Iran's neighbors, particularly the Soviet Union.

No details have et emerged on the extent of a reported sharp cutback, but U.S. business sources say it could be as drastic as just abandoning the project. Other U.S. defense contractors with somewhat clouded futures here include Lockheed, Raytheon, Northrop, Grumman. General Dynamics and Hughes Aircraft.

More crucial to Iran's future than the payments owed to U.S. companies will be the conutry's attitude toward its massive foreign borrowings, analysts say.