The Justice Department yesterday urged the Civil Aeronautics Board to halt additional purchases of National Airlines stock by Texas International Airlines and Pan American World Airways until the board decides whether to approve either of their merger proposals.

TXI and Pan Am both have tentative board approval to acquire up to 25 percent of National's stock as long as it is placed in trusts which prohibit their exercising any control over National.

TXI has disclosed that it has purchased shares giving it 20.1 percent of the stock, while Pan Am has acquired at least 4.8 percent of National's shares.

The department's petition argued that the CAB had raised questions about the proposed mergers which would have justified the board in prohibiting any additional purchases of stock by either airline, as a court faced with similar questions about non-airline mergers could have done. It suggested that the board should now freeze holdings at present levels.

For the future, the Justice Department also recommended that the board set up a regulatory mechanism to require prior notification of airline takeover attempts just as Justice and the Federal Trade Commission are now alerted in advance to impending stock purchases or mergers.