NATO is making good progress in building up the battalions being deployed in eastern European allies badly rattled by a more assertive Russia, alliance head Jens Stoltenberg said Wednesday.

NATO leaders endorsed plans at their Warsaw summit in July to rotate troops into the three Baltic states and Poland to reassure them they would not be left in the lurch if Russia was tempted to repeat its Ukraine intervention.

"I am actually very inspired by the meeting today because so many nations made very, very firm and concrete decisions and announcements of their contributions to the four battalions," Stoltenberg told reporters at a NATO defence ministers meeting in Brussels.

Such commitments were "a transatlantic demonstration of rock solid support for our allies," he said.

At this rate, deployment was on track for early next year, as planned, he added.

Britain, Canada, Germany and the United States agreed in July to lead the battalions of some 1,000 troops each, with the other 24 NATO allies expected to provide different components -- transport, communications and medical units.

Stoltenberg said Albania, Italy, Poland and Slovenia would contribute to the Canadian battalion deployed in Latvia.

Germany's battalion in Lithuania will be supported by Belgium, Croatia, France, Luxembourg, the Netherlands and Norway.

Denmark and France will help Britain's unit in Estonia, while Romania and Britain again will contribute to the US-led battalion in Poland.

Stoltenberg said this showed that the battalions will be "robust, multinational and combat ready."

Their deployment will send "an unmistakable message -- NATO stands as one. An attack on one ally will be considered an attack on all," he said.

The secretary general, a former Norwegian premier, noted that the allies were also meeting commitments on a Romanian-led unit which will help bolster security in the Black Sea region in the same way as in the Baltics.

He repeated that NATO was responding in a measured fashion to a Russian military build-up and increased activity around NATO's borders.

The alliance would take all the measures it deemed necessary but also remained open to dialogue with Moscow, which was especially important at times of tensions, he said.