The Interstate Commerce Commission ordered the Southern Pacific Railroad yesterday to cease purchases of stock in the Seaboard Coast Line railroad for at least 70 days.

The action is the latest round of a topsy-turvy attempt by SoPac to merge with the southern railroad. The ICC also asked SoPac to tell the commission, within 14 days, just how much Seaboard stock it owns or has in trust.

SoPac said yesterday it has acquired 9.6 percent of Seaboard's stock (1.4 million shares), making it Seaboard's largest single stockholder.

The commission action came in response to a complaint filed by Seaboard on July 21, alleging that SoPac was trying to take control of Seaboard without the required ICC approval for such an action.

The agency monitors railroad mergers and acquisitions for possible antitrust violations.

But in its filings with the ICC. SoPac has said it has no immediate plans to seek ICC approval of a takover. The San Francisco-based company has, however, indicated that there are long-term plans to acquire the company and that it will eventually apply for permission to acquire Seaboard, which would give SoPac a valuable direct link from the West Coast to the Gulf Coast.