The Civil Aeronautics Board yesterday granted Pan American World Airways long-sought authority to carry local passengers on five domestic segments of its international routes, including a daily flight from Washington to New York.

Starting Dec. 1, Pan American will be able to fill up the empty seats on international flights operating between New York and Philadelphia, Seattle and Portland, Miami and Tampa, and San Francisco and Los Angeles.

Although "fill-up" rights were also awarded to Braniff Airways and Northwest Airlines in three markets, for Pan Am yesterday's action was a significant step forward in its long quest to add some of the United States to its "world."

Except for the granting of fill-up authority to Pan Am last fall to carry domestic traffic between Detroit and Boston on daily flights bound for and back from London. Pan Am has no other authority to carry passengers on flights between the 48 contiguous states. Over the past few years, it has variously sought - with little success - outright domestic routes and fill-up rights, arguing that domestic routes would improve its efficiency and balance its system.

It also has argued that it ought to get some domestic routes out of fundamental "fairness," noting that the CAB over the past 30 years has continued to award new international routes to domestic carriers to compete with Pan Am, but had not in turn awarded Pan Am any domestic routes.

Pan Am carried 1.3 million empty seats in 1975 on flights in its nine domestic segments of international flights: there were more than 975,000 unused seats on the segments for which it won fill-up rights yesterday.

"It's certainly a sensible first step," John Krimsky, Pan Am's vice president for federal affairs, said yesterday. He said Pan Am was "encouraged" by the board's action and hopes it would consider Pan Am's request for fill-up rights on some of its longer-haul domestic sectors, such as New York-Dallas.