Central banks have always watched currency levels, and their interest rate moves affect them. But most have avoided explicitly tying monetary decisions to foreign exchange out of fear of being called manipulators, which could bring geopolitical risks.

The lines are blurry, but that designation is usually reserved for political authorities that directly buy and sell currencies to change their prices and gain a competitive edge. The United States labeled China a manipulator last week.

But as ways to stimulate domestic activity with monetary policy look increasingly tapped out, less explicit attempts to guide currency prices — by changing rates and buying bonds — might become a more important part of central bankers’ playbooks. Jeremy Stein, a former Fed governor, warned that if central banks increasingly competed on foreign exchange, the risk fell short of a full-blown currency war but could touch off a “sort of competitive easing” — a rush to cut rates first to reap the currency benefits.

The Bank of Japan took the rare step of tying currency to a potential monetary policy move in February when its leader told lawmakers that it could be forced to enact additional stimulus if the yen kept strengthening. Otherwise, he argued, the nation’s dangerously low inflation might turn even lower.

When Australia’s central bank cut rates in June, according to meeting minutes, officials there recognized that “the main channels through which lower interest rates would support the economy were a lower value of the exchange rate” and lower household borrowing expenses.

In a world with already low interest rates, “the international environment becomes more important, because depreciation of the currency is the one remaining option,” said Joseph Gagnon, an economist at the Peterson Institute for International Economics and formerly the Fed. “And surely that has problems, because currency depreciation is a zero-sum game: Anything you get, the other guy loses.”

Using rates to control currency levels could prove costly. Any stimulus that a central bank can eke out of devaluation comes at a direct expense to its trading partners, and is likely to be short-lived before other countries cut rates or buy bonds to compete.