BRUSSELS—The North Atlantic Treaty Organization is poised to approve an expansion of its troop deployments on the alliance's eastern flank next week, U.S. and alliance officials said Friday.

NATO officials in Brussels met Friday to pull together a proposal for expanding the NATO mission in Eastern Europe, a measure expected to be approved by defense ministers at a meeting in Brussels next week.

Officials said the size of the force or precise contributions won't be determined until later this year. But that defense ministers are likely to approve a NATO mission that would oversee and enlarge what are currently bilateral deployments to the Baltic States and Poland.

Russia has repeatedly said any additional deployments are deeply destabilizing and a threat to its security. Officials have demanded that NATO hold talks with Moscow aimed at putting a precise limit on the number of troops the alliance can deploy to the East.

Having the NATO flag flying over even a small troop presence in the East will have an important deterrent effect, and is a key step to bolstering alliance defenses, according to NATO officials.

The announcement this week by the U.S. that it plans to spend an additional $3.4 billion on European defense, money that would pay for a persistent presence of an additional 3,000 forces in Europe, was timed in large measure to spur forward the alliance's discussions on reinforcing the east.

In Amsterdam on Friday, NATO Secretary General Jens Stoltenberg praised the U.S. announcement, noting that it would "fund more troops in the Eastern part of the alliance."

The new American rotational forces, made up of armored brigades deploying from the U.S. to Europe, would form the pool from which the U.S. could contribute additional forces to Poland and the Baltic States.

U.S. officials said they hope that the U.S. announcement will spur other countries to increase their own contributions to building up the NATO troop presence in the east. U.S. officials wouldn't say how many additional troops they want to see Europe contribute to bolstering deterrence.

President Barack Obama, a U.S. official said, "is calling on allies to step up and do more. The U.S. is leading by example."

Maria Zakharova, the spokeswoman for the Russian foreign ministry, said the U.S. announcement of a boost in European military spending was carried on in an "aggressive manner" and was aimed at containing Russia.

"As we hear these announcements from the United States, all of this creates grounds to roll out military plans against Russia and take real practical steps to push military infrastructure closer to Russia's borders," she said Thursday.

The U.S. currently contributes a company—about 150 soldiers—in each of the Baltic States and Poland. A U.S. official said Friday that the persistent NATO presence on Russia's borders could rise to a brigade—roughly 4,000 troops, without violating existing agreements.

In addition to the U.S. deployments, Britain and Germany regularly deploy troops for training exercises in the Baltic States and Poland. Last month, Britain announced the deployment of some 800 troops to Poland for exercises over the course of the year.

The NATO-Russia Founding Act, which was agreed to as the alliance began to expand eastward, promises to avoid permanently positioning substantial combat troops in Eastern Europe. As a result, the U.S., Britain and Germany oppose any large, permanent NATO bases in the east, but are amenable to a smaller force.

The U.S. official said after defense ministers formally agree that deterrence needs to be strengthened and "enhanced forward presence is needed," member states would work to generate a set of troop requirements ahead of the July summit of top alliance leaders.

In addition to paying for the additional rotational forces, the expanding U.S. spending on European Defense will help pay for creating additional pre-positioned stocks of heavy equipment, which would allow the U.S. to quickly fly in a fourth brigade to Europe in a time of crisis, officials said.

"The objective here is deterrence," said the U.S. official. "In a crisis you are going to fly this brigade over…you can do that fast."

Thomas Grove in Moscow contributed to this article.

Write to Julian E. Barnes at julian.barnes@wsj.com

(END) Dow Jones Newswires

February 05, 2016 09:25 ET (14:25 GMT)