NATO Secretary-General Jens Stoltenberg said on Friday that the presence of American troops in Romania showed the United States' commitment to the security of Europe, as more soldiers arrived at an airbase.

Speaking alongside Romania's president, Stoltenberg also reiterated his concerns about a possible Russian "invasion" in Ukraine and said the alliance would consider a longer-term presence in south-eastern Europe in the coming months.

"We continue to reinforce our eastern flank," Stoltenberg said, stressing that NATO's actions were defensive.