Europe’s top economic official has cautioned the U.S. and Treasury Secretary, Steven Mnuchin, about talking down the dollar’s exchange rate. This would help U.S. exporters but could make life harder for Europe and other trade partners.

European Central Bank President Mario Draghi didn’t mention Mnuchin by name but was quick to note Thursday that global leaders had agreed for years that “we will refrain from competitive devaluations.”

Draghi’s swipe came after Mnuchin said this week that “obviously a weaker dollar is good for us as it relates to trade and opportunities.” Those remarks broke with a 20-year U.S. policy of speaking in favour of a strong dollar.

Read: Hard year ahead for U.S. dollar: Desjardins

Such comments matter because a weaker dollar can help U.S. exporters by making their goods cheaper against foreign competition.

But it’s a zero-sum game: a fall in the dollar means a rise in a corresponding currency—the euro rose to a three-year high of $1.25 upon Mnuchin’s remarks. The higher euro can become a headache for Europe as it can hurt its exporters and weigh on inflation, which is already worryingly low.

In his comments on Thursday, Draghi cited the repeated joint statements by international finance officials in which they commit to refrain from lowering their currencies’ exchange rates to gain trade advantage at the expense of other countries.

Draghi added that recent swings in the euro exchange rate with the dollar—mostly up in the past few days—were “a potential source of instability” that required “monitoring.” Draghi noted that the exchange rate wasn’t a policy goal for the ECB but had to be considered in its deliberations about what to do about inflation.

Read: Hard year ahead for U.S. dollar: Desjardins

Exchange rates are determined by many factors, including trade flows and central bank policies. Rates these days are not set by governments but by global foreign exchange markets where major currencies are bought and sold. Official comments can, however, have an impact.

One risk, however, is that other countries might retaliate by taking measures that lower their currency.

“The ECB refrained from aggressively pushing back against the strength of the euro at their monetary policy decision […] but also warned the U.S. against opening a Pandora’s box of competitive devaluation,” said William Adams, senior international economist at the PNC Financial Services Group. “Put simply, Draghi is warning that foreign countries could follow the U.S. in a race to the bottom if the U.S. tries to devalue the dollar.”

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ECB leaves interest rates unchanged

Draghi made his comments at a news conference following a decision by the bank’s 25-member governing council to leave its stimulus programs and interest rates unchanged. The bank gave little additional hint about whether its 30 billion euros (US$36 billion) in monthly bond purchases would run past September.

Currently, the bank says the purchases will continue at least through September and longer if necessary, leaving the exact end open. The purchases pump newly created money into the economy to raise inflation and growth in the wake of the 19-country eurozone’s crisis over high debt in member states like Italy and Greece.

Stimulus withdrawal has been much discussed in markets because the eurozone economy is growing strongly. Germany’s Ifo index of business sentiment matched its record high in January, and surveys show business activity is expanding rapidly.

The eurozone is expected to have grown 2.4% last year, while unemployment has fallen to 8.7% from a high of 12% in 2013.

The bank left its key short-term interest rate benchmark unchanged at a record low of zero. Its rate of minus 0.4% on deposits it takes from commercial banks was also unchanged. The negative rate is a penalty aimed at pushing banks to lend the money rather than let it pile up risk-free at the ECB.