The Reagan administration yesterday extended to Syria its export controls on eight compounds used to manufacture chemical weapons, saying it was concerned that the Syrians now have the capability to produce such weapons and are helping Iran do the same.

The United States has already banned export of the same eight compounds to Iran and Iraq. State Department spokesman Bernard Kalb said, "Iran has not used chemical weapons but their continued use by Iraq and the potential for Iranian use is a cause of concern."

Other U.S. and Israeli sources said Syria is producing chemical weapons, possibly at a plant in the Damascus area. The sources were uncertain whether Syria has passed on its technology or chemical weapons to Iran.

Syria and Libya have long provided Iran with military supplies for its war against Iraq in return for low-cost Iranian oil. These two countries are the only Arab nations aiding Iran, which is a Moslem but not an Arab Middle Eastern country.

The State Department said yesterday it does not believe any U.S. companies are exporting the eight chemical compounds to Syria. The export control was described as "a precautionary measure."

Kalb said the decision was made in accordance with the U.S. policy of seeking to halt the spread of chemical weapons round the world and the knowledge that Iraq and Iran, at war now for almost six years, both have "active, on-going programs" to develop such weapons. He also said the measure reflected the U.S. effort to remain neutral in the war.

One State Department official said the main reason for the control on chemical exports on Syria is to give the administration leverage against Western Europe to take similar action in halting assistance to Syria from European companies. Another official said the decision has been moving through the bureaucracy for at least six months.

In March 1984, the United States placed controls on exports to Iraq and Iran of five compounds used in making chemical weapons. That followed the confirmed use of poison gas by Iraq, as well as its reported use of a deadly nerve gas, Tabun. Later, the export ban was extended to include a total of eight chemical compounds.

U.S. intelligence reports that Syria was developing a chemical weapons capability first became known publicly in 1983.

In March, a U.S. source briefing reports on the status of U.S.-Soviet arms control negotiations disclosed that Syria was producing chemical weapons on the basis of technology obtained from a number of sources that were not disclosed.