Republican presidential candidate Mitt Romney laid out a hawkish foreign policy in a speech in South Carolina on Friday, saying he would boost defense spending and expand the size of the Navy while deferring to military commanders on how long to keep U.S. troops in Afghanistan.

Many of the former Massachusetts governor's positions will set him apart from some of his rivals in the race for the Republican nomination—and from others within his own party—who want the U.S. to begin withdrawing from Afghanistan and say the Pentagon cannot be immune from the drive to cut federal spending.

Mr. Romney, who leads the field of GOP candidates in most public opinion surveys, spoke before 200 or so cadets at the Citadel military academy in Charleston. His comments were the most expansive of any Republican candidate to date on U.S. military spending and the nation's posture abroad.

He proposed a dramatic increase in naval shipbuilding, to as many as 15 new ships a year, up from the current rate of nine a year, as well as the deployment of a full national ballistic-missile defense system.

The speech came a day after Mr. Romney proposed increasing U.S. active-duty forces by 100,000 people and boosting noncombat defense spending to 4% of gross domestic product, up from about 3.8% now. That increase would amount to roughly $30 billion more in defense spending a year.