NATO will continue to reinforce its eastern flank amid a Russian military build-up on Ukraine's border but will also consider a longer-term presence in the Black Sea region, Secretary-General Jens Stoltenberg said on Friday.

Speaking alongside Romania's president at a military airbase in eastern Romania where additional U.S. troops are being deployed, Stoltenberg said the presence of American troops underlined the U.S. commitment to the security of Europe.

"There is an ongoing strengthening of our NATO presence in the eastern part of the alliance," Stoltenberg said. "We've also increased the readiness of NATO's response force. These troops are in their home bases, but they can be quickly deployed anywhere in the alliance if needed."

"On top of that we are considering more longer-term adjustments in our posture, and that includes establishing battlegroups in the southeast of the alliance, meaning Romania but also other countries around the Black Sea region."

France has offered to be the lead nation of a future NATO mission in Romania, which could see about 1,000 troops from various countries, and battlegroups will be discussed at a NATO defence ministers meeting next week.

However, Stoltenberg said a decision about a longer-term presence could be made in the spring.

"I have repeatedly pleaded for a more consistent presence of NATO, the United States and other allies in the Black Sea region," Romanian President Klaus Iohannis said.

The Stryker squadron of 1,000 U.S. service members relocating from Vilseck, Germany to Romania will add to the 900 soldiers Washington currently has rotating in the Balkan country. Italian, German and Polish troops are also in Romania.

On Friday Stoltenberg also reiterated his concern about a possible Russian invasion in Ukraine.

"There is a risk of a full-fledged invasion, but there's also a risk of other types of aggressive actions, including attempts to topple the government in Kyiv, hybrid cyber-attacks and many other types of Russian aggression."

Reuters

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