The Justice Department said yesterday that the acquisition by United Airlines Inc. of Pan American World Airways' Pacific routes may violate federal antitrust laws and deserves further investigation.

In comments filed with the Department of Transportation, Justice "took no position on the proposed acquisition, but indicated that its preliminary analysis suggests that it is possible that the acquisition could have an adverse effect on competition," a spokeswoman said.

The analysis said the sale may hurt competition for air passenger service between the United States and two locations, Japan and Hong Kong, which are now served by both airlines.

Pan Am, which pioneered air travel to the Pacific, agreed to sell its Pacific Division to United for $750 million, a transaction that requires the approval of the Department of Transportation and President Reagan. United would gain 17 jumbo jets and Pan Am's routes to China, Australia, New Zealand, Hong Kong, Singapore, Taiwan, Korea, Thailand and the Philippines.

Acting Assistant Attorney General Charles F. Rule said in a statement that Justice "must further investigate the acquisition" to determine its effect on competition, "as well as the possibility that if competitive problems are found to exist, they can be cured by appropriate route spinoffs."