WASHINGTON (AP) _ The State Department called on the Nicaraguan government Thursday to rescind new restrictions that force U.S. diplomats who leave Nicaragua to delay their return for three weeks.

Sandinista authorities informed the U.S. Embassy in Managua of the change in procedures last week during a period when the two countries were embroiled in a dispute over a proposed visit - since canceled - to the United Nations by President Daniel Ortega.

The new regulations require embassy personnel leaving Nicaragua to seek re- entry visas only outside Nicaragua. In addition, a waiting period of 15 working days was imposed.

The State Department said the shift means that diplomatic personnel who must leave Nicaragua for any reason, including official business, family emergencies or medical treatment, will not be able to re-enter Nicaragua for three weeks.

Beforehand, re-entry visas could be obtained prior to departure from Nicaragua with five days advance notice.

''We believe that the Nicaraguan government should rescind this latest action,'' a department statement said, adding that ''appropriate responses'' are being considered.

It also noted that the new measures aggravate the existing personnel shortage at the embassy. Last July 11, the Sandinistas expelled eight U.S. diplomats on grounds they were interfering in Nicaragua's internal affairs. The United States responded by expelling eight Nicaraguan diplomats from Washington.

Neither side has allowed the other to send replacement envoys.

Last week, Ortega canceled plans to visit the United Nations because, he said, the United States would not allow a full delegation to accompany him. The State Department contended that visas applications of most of the official party were turned in too late.