President Carter yesterday set an $8.6 billion ceiling on U.S. arms sales outside the NATO-Japan-Pacific ally bloc for fiscal 1978 - a cut some sources said might be felt chiefly by Israel.

Despite the 8 percent reduction in sales to the nation's largest category of arms clientele - mainly Middle Eastern nations - overall foreign military sales will still increase by $2 billion in fiscal 1978.

The announcement came as administration spokesmen were testifying on arms sales in Congress, where critics claim Carter has broken campaign pledges to reduce the U.S. arms trade overseas.

White House officials said military sales to allies - NATO countries, Australia, New Zealand and Japan - are expected to be about $1.5 billion in fiscal 1978, which will end Sept. 30, compared with $1.2 billion this year.

In addition, they said, Carter has authorized $8.6 billion in arms sales to other countries, most of them in the Mideast. In fiscal 1977, which ended last Sept. 30, the figure was $9.3 billion.

But the United States also expects to sell about $3 billion in military equipment and services other than weapons in fiscal 1978, compared with $900 million the year before.

Overall, foreign military sales in 1978 will be about $13.2 billion, compared with $11.2 billion last fiscal year.

In a statement released by the White House, Carter said any larger cut in the arms sale ceiling "would violate commitments already made, including our historic interest in the security of the Middle East."

Unlike previous arms policy statements, however, this one did not specifically mention the U.S. commitment to maintaining the security of Israel.

Administration officials said 80 percent of all U.S. arms sales outside NATO, Japan, New Zealand and Australia go to the Middle East - chiefly to Israel and Iran.

They said they expected any future cuts to come in that region.

And because such major items as the $2 billion early warning radar aircraft system for Iran are committed, they said they expected Israel would feel the greatest impact.

Israel buys about $1.5 billion a year in U.S. arms. One government source said the cut in sales will probably mean that Israel will have to "stretch out" some purchases, such as F-15 and F-16 aircraft.

This official said Israel would still be buying the same number of planes, but the deliveries would be stretched out so the administration can show an annual cut in its total worldwide arms sales.

On the other side of the arms sale issue, Carter said that a "smaller reduction would neglect our responsibility to set an example of restraint that others might follow."