Key Western countries today offered a $1 billion credit bonus to Prime Minister Bulent Ecevit in an effort to help bail debt-ridden Turkey out of its mounting economic difficulties.

The offer, relayed by visiting U.S. Deputy Secretary of State Warren Christopher, fell far short of Turkey's expectations, however. There was no immediate Turkish reaction, but the step is unlikely to halt Ankara's drift toward nonalignment.

The decision to exrend additional assistance to Turkey was taken by President Carter and the leaders of West Germany, Britain and France at their Guadeloupe wummit last week. It was apparently prompted by Western concern over the deteriorating political and economic situation in the so-called Northern Tier -- stretching from the Bosphorus to the Himalayas -- as a block to Moscow's expansion to the oil-rich Persian Gulf.

The $1 billion credit bonus ove the next five years probably would be handled through a special arrangement with the International Monetary Fund. It would come on top of regular Western commitments. The United States has already agreed to give Turkey $225 million in military and economic aid in 1979.

But the left-of-center Ecevit government, which inherited a weak economy when it took power a year ago, has suffered additional economic blows as a result of an influx of Turkish workers from West Germany and the continued high cost of Turkey's military occupation of northern Cyprus.

Turkish workers have been returning in substantial numbers since the Bonn government halted extension of work permits to foreign workers in an effort to cut West Germany's unemployment rate. As a result, Turkey's already high unemployment levels have increased.

One of the byproducts of the economic predicament has been an increased level of social unrest in Turkey as well as a rising cycle of terrorist attacks which have claimed an estimated 1,000 lives in 1978.

Ecevit associates have been saying that they need $15 billion in foreign credits to finance their 1979-83 economic development plan. They were hoping for about $5 billion from the Guadeloupe summit.

Thus the $1 billion offer may be viewed here as the latest in a series of economic disappointments for the Turks.

Foreign Minister Gundoz Okcun, after a recent tour of Western Europe and North America, said he found Turkey's NATO allies reluctant to support the failing Turkish economy.

Moreover, the Turks feel they have been outmaneuvered by their chief rival, Greece, whose application to join the European Common Market was recently approved while Turkey is not likely to join for some time.

Opposition leader Suleyman Demirel charged in a recent interview that although the United States had repealed its ban on military aid to Turkey -- imposed following the 1974 Turkish invasion of Cyprus -- there was still an economic embargo in force against the Turks.

"You won't find anyone who will confirm this," Demirel said, "but the question of the arms embargo and the economic embargo are inseparable. What the United States is saying is: 'You either solve the Cyprus problem the way we want it or we will make you.'"

Western sources said implicit in the Western offer of aid was that Turkey should come to terms with Greece over Cyprus, where the Turks presently occupy one-third of the island. The Cyprus dispute, coupled with the anti-shah ferment in Iran, have seriously weakened the West's southeast defenses.

In addition to its NATO membership, Turkey is also a member with Iran of the Central Treaty Organization (CENTO).

But their growing disillusionment with the West has led the Turks to seek closer economic relations with the Soviet Union. The Soviet Union has appeared to have responded sympathetically to Turkey's economic plight, granting $300 million in credits to Ankara.

Turkey is now one of the largest recipients of Soviet aid outside the Soviet bloc.

Turkey has been forced by lack of foreign exchange to delay payment of $5 billion of outstanding debts; its inflation is running at 70 percent and unemployment at more than 20 percent.