The State Department yesterday warned American citizens against any but essential travel for the time being to 11 Islamic countries where U.S. diplomatic staffs are being reduced as a security measure.

Spokesman Hodding Carter described the travel advisory for private citizens, like the reduction of official personnel announced Monday, as a "prudent, precautionary. . . temporary step."

State Department officials denied that any specific threat against embassy personnel or American travelers led to the government decision. They said the high tension arising from the Iranian crisis, especially the strident appeals to the Islamic masses by Ayatollah Ruhollah Khomeimi, made the precautions advisable.

A report on the actions being ordered was made available to the State Department press office about noon Monday, with instructions that the information was to be given out only if reporters asked. Following a news agency inquiry and State Department admission of the embassy staff reductions late Monday, offficials refused to provide a list of the posts which are affected.

The countries on the list, as divulged to the press yesterday, are:

Iraq, Kuwait, Bahrain, Qatar, the United Arab Emirates and Oman. Including Iran, where 49 official Americans are held hostage and private Ameicans already have been warned to leave, these are all the countries of the oil-rich Persian Gulf except for Saudi Arabia.

The Shiite branch of Islam, the sect of Khomeini and most Iranians, makes up a majority in Iraq. Shiites, including expartriate workers from Iran, make up signifcant minorities in the other Gulf countires. Saudi Arabia was not included because of its relatively strong security forces and U.S. sensitivity to its position as the world's top oil exporter.

North Yemen, an unstable country where the United States undertook a Saudi-financed $500 million military aid program last March

Lebanon and Syria, where security for Americans is tenuous due to internal strife. The Lebanese government has limited control in most areas, and Lebanese Shiites have been agitated for 15 months because of the mysterious disappearance of their leading imam. Both anti-Americanism and internal instability have grown recently in Syria.

Libya, where one of the most radical governments in the Islamic world has allied itself vocally with Iran, even while officially disapproving of the taking of diplomats as hostages.

Bangladesh a predominantly Islamic country in South Asia where security forces might not be adequate to handle large scale disturbances.

Turkey and India, both of which have recently experienced demonstra- tions against American diplomatic installations, were excluded from the list on grounds that large and responsive government security units probably can deal effectively with problems which may arise.

For the time being, North African countries such as Tunisia and Islamicc countries in Africa were also excluded. State Department officials said more countries might be added to the precautionary list, suggesting that additional measures are being studied.

A total of some 410 to 440 official Americans are on duty in the 11 countries named yesterday, according to the State Department. No estimate was available of the number of officials likely to be classified as "nonessential" and thus eligible to depart temporarily at government expense nor the number of dependents likely to depart.

Travel advisories against nonessential visits by private Americans were issued previously for Parkistan and Afganistan as well as Iran, according to State Department officials.