One was a fear — nourished in part, some sources said, by Henry A Kissinger, then the President’s national security adviser — that Daniel Ellsberg, who said he turned over the Pentagon papers to the press, might pass on to the Soviet Union secrets far more important than any information contained in the Pentagon study of the Vietnam war.

Specifically, the sources said, the White House feared that Dr Ellsberg, a former Rand Corporation and Defense Department official, may have been a Soviet intelligence informer who, in the weeks after publication of the Pentagon Papers in June, 1971, was capable of turning over details of the most closely held nuclear targeting secrets of the United States, which were contained in a highly classified documents known as the Single Integrated Operation Plans, or SIOP.

The second major concern was that a highly placed Soviet agent of the KGB, the Soviet intelligence agency, operating as an American counterspy, would be compromised by continued inquiry by the special prosecutor and the Senate Watergate committee into the Ellsberg case. The agent informed his FBI contact that a set of the Pentagon papers had been delivered to the Soviet Embassy in Washington shortly after a Federal court had ordered The Times to stop printing its series of articles on the papers.

—Seymour Hersh
The President and the Plumbers (9 Dec 1973)

H/T: Trevor Timm, Before Snowden, Nixon Admin Pioneered Evidence-Free “Russian Spy” Smears Against Daniel Ellsberg