The State Department, after what its spokesman called "a thorough investigation," yesterday denied a report that an official in State's Operations Center mistakenly had given approval for the police assault that led to the death of U.S. Ambassador Adolph (Spike) Dubs in Kubul, Afghanistan.

Steve R. Pieczenik, a former deputy assistant secretary of state who made the charge in an article in the Outlook section of Sunday's Washington Post, said following the State Department denial, however, that "detailed evidence" in his possession backs up his original statement.

Pieczenik said he was present in the State Department Operations Center in the early morning of Feb. 14, 1979, and a witness to the telephoned instructions to Kabul by an official, whom he declined to name, approving an assault by Afghan police on the hotel room where Dubs was being held prisoner by unidentified terrorists.

Pieczenik added that he has complete notes, taken at the time, of what happened, and that he had reported the mistaken instructions to two senior officials of the State Department the same day. He declined to name the officials.

According to Pieczenik, the approval for the assault on the hotel room was countermanded, but too late to save the life of Dubs, who was killed in a hail of gunfire.

State Department spokesman William Dyess, responding to reporters' questions, said a review of State Department logs of the incident and interviews with the officers directly concerned, both here and in Kabul, led to a departmental conclusion that the allegation is "not correct." Dyess said all those interviewed agreed that "throughout the incident written and oral instructions stressed the need for restraint and moderation as well as the need to avoid any precipitous assault." The spokesman said it had "not come to light" that Pieczenik reported the "mistaken instructions" to higher-ups at the time.