WASHINGTON — The Trump administration announced Monday that it had decided against imposing any sanctions on countries that buy Russian military equipment, saying that a new law was already deterring billions of dollars in such purchases.

The law required that sanctions be imposed against large purchasers of Russian arms, but it granted exceptions for a variety of reasons. The administration explained the exceptions it was citing to members of Congress in a classified briefing on Monday.

“We estimate that foreign governments have abandoned planned or announced purchases of several billion dollars in Russian defense acquisitions” since the enactment of the law in August, said Heather Nauert, the State Department spokeswoman.

Defense deals are often years in the making, so last year’s law, called the Countering America’s Adversaries Through Sanctions Act, is only just beginning to have an effect, an administration official said.