The Senate Foreign Relations Committee has refused, at least temporarily, to approve six of President Reagan's nominees for vacant ambassadorships, thereby requiring that all six be renominated after Jan. 1 and begin anew the lengthy process of winning Senate confirmation to take up their ambassadorial duties.

Congressional sources attributed the delays to a variety of reasons, adding that all could be described generally as the result of committee members saying they wanted more information about the nominees.

The situation has focused new attention on the tendency of some senators, mostly but not exclusively conservative Republicans, to use the ambassadorial confirmation process as a weapon for imposing their foreign policy views by defeating or holding hostage the administration's nominees.

Of seven nominations considered by the committee last Thursday, only that of Henry Anatole Grunwald, retiring editor-in-chief of Time Inc., to be ambassador to Austria was approved and forwarded to the Senate for a confirmation vote.

The remaining six were held up for further consideration. However, because the Christmas recess means that Congress will not reconvene in this calendar year, the nominations will lapse and will have to be resubmitted next year. The six include four professional diplomats and two outside political appointees.

The career Foreign Service nominees are John R. Davis Jr. to Poland, Charles F. Dunbar to the Yemen Arab Republic, April C. Glaspie to Iraq and Richard H. Melton to Nicaragua. The outsiders are Milton Frank, a California public relations executive, to Nepal, and Bill K. Perrin, a Texas businessman, to Cyprus.

The delays mean that the ambassador's offices in these posts are likely to remain empty for several more months, and some are in countries where the State Department believes it is important to have a fully functioning ambassador in place. These include Poland, where Davis has been acting ambassador since 1983 during a period of strained relations; Iraq, whose war with Iran has caused the administration to commit massive naval forces to the Persian Gulf, and Nicaragua, where the United States has been backing a guerrilla war against the Marxist Sandinista government.

Congressional sources said it was difficult to sort out which committee members had blocked action on specific nominees, although they said that most appeared to have run up against complaints from Sen. Jesse Helms (R-N.C.) that the nominees or the State Department failed to provide sufficient information about their backgrounds or policy positions. Helms, the committee's senior minority member, has waged unceasing war against administration appointees whom he regards as insufficiently conservative and anticommunist.

The sources added that Melton's nomination apparently was held up by liberal Sen. Christopher J. Dodd (D-Conn.), chairman of the Western Hemisphere subcommittee and a strong critic of the administration's Central America policies. Melton is closely associated with Elliott Abrams, assistant secretary of state for inter-American affairs; Dodd has barred Abrams from appearing before the subcommittee because he has admitted misleading Congress in testimony about the covert war in Nicaragua.

In another action, the committee voted 11 to 8 to set up a temporary subcommittee to consider ways of improving the War Powers Resolution, which has been a subject of contention between Congress and successive presidents since its passage during the Vietnam war. The subcommittee will be headed by Sen. Joseph R. Biden Jr. (D-Del.).