The United States, concerned that Jordan is now accepting supplies intended to help neighboring Iraq fight Iran, cautioned yesterday that such lack of neutrality could cause the Iran-Iraq war to spread to other countries.

Prompting the concern was Jordanian King Hussein's decision to permit ships that cannot enter Iraqi ports because of the fighting to discharge their cargos at Jordan's Red Sea port of Aqaba. The supplies apparently then are to be transshipped across Jordan to Iraq.

State Department sources said that U.S. objections to Hussein's helping Iraq in this manner have been communicated to Jordan "clearly and emphatically" but with no effect. But, the sources added, the level of Jordanian involvement in the conflict appears to be relatively low at this point, and they stressed that the Carter administration has not threatened to take any action against Jordan if it persists in helping Iraq.

According to the sources, a number of Iraq-bound cargo ships from several countries that were unable to land have been diverted to Aqaba. The sources said these ships were carrying food, building materials and possibly some spare parts of a military nature.

State Department spokesman John Trattner refused to identify the origins of any of these ships except to say that none had come from the Soviet Union, which is Iraq's principal military supplier.

However, the sources said ships with supplies apparently intended for Iraq had been identified putting into Aqaba from India, Lebanon and some East European countries. The sources cautioned, though, that these appeared to be vessels engaged in normal commerce with Iraq and said their origins should not be interpreted as a sign that the countries in which they are registered are supporting Iraq in the war.

One question puzzling U.S. officials was why Hussein, normally regarded as a leading Arab moderate and one of America's closest friends in the Arab world, has elected to play such an up-front role in the conflict. Secretary of State Edmund S. Muskie, talking briefly with reporters yesterday, gave one speculative view.

"It's obvious that Jordan feels the importance of its ties to Iraq and perhaps the importance of Iraq's future influence in the Gulf area," he said.

The U.S. fear that actions such as Jordan's could have the effect of bringing other countries into the conflict was expressed indirectly by Deputy Secretary of State Warren M. Christopher during a speech yesterday in Boston. He said "the intensity and scope of the fighting has almost certainly exceeded the expectations of everyone invloved. So once again we have seen how easy war is to start -- and how difficult to contain and conclude."

In his speech, Christopher also reaffirmed the neutrality proclaimed by the United States when the fighting began two weeks ago and called on the Soviet Union to continue maintaining a similar neutrality. However, department sources privately called attention to one part of Christopher's speech as a veiled warning that the United States might abandon its neutral source if the war spreads to include important U.S. allies such as Saudi Arabia with its vital oil supplies.

In setting forth U.S. aims, Christopher specified that one is "to prevent the conflict from expanding in ways which threaten the security of the region." He added that "we will also respond to requests for assistance from non-belligerent friends in the area who feel threatened by the conflict."

The United States already is sending radar command planes to Saudi Arabia, and the sources said Christopher's statement should be read as a warning that Washington is prepared to increase the level of U.S. military involvement in the Gulf region if necessary.

Trattner, responding to questions about Jordan, said Hussein's government, which depends on the United States for most of its arms, had been reminded that no U.S.-supplied weapons can be sent to another country without Washington's approval. He also noted that there is no evidence so far of Jordan providing any weapons or troops to Iraq.

In June, the United States agreed to sell Jordan 100 advanced M60A3 tanks equipped with night-vision sights and to consider the sale of an additional 100 tanks at a later date. Since Jordan has sided publicly with Iraq, Sen. Richard Stone (D-Fla.), a strong supporter of Israel, has called for reassessing the tank deal.

However, U.S. officials said yesterday that, given the need to cultivate Hussein's continued friendship for a variety of Midwest policy reasons, there is no intent or reason at this point to use Hussein's support of Iraq as a pretext for disturbing U.S.-Jordanian relations. They added that Jordan's involvement in the conflict would have to become much greater and more serious for the situation to reach that point.