By Adrian Croft

BRUSSELS (Reuters) - NATO defense ministers are expected to agree next week to set up command units, staffed with national and NATO soldiers, in six eastern European allies as part of a new strategy in response to the Ukraine crisis, NATO's chief said on Friday.

Creating the units in Poland, Romania, Bulgaria and the three Baltic countries is one of the most concrete steps NATO has taken to show that the alliance will protect its eastern allies, some of which are nervous about Russian intentions following Moscow's annexation of Ukraine's Crimea region.

NATO Secretary-General Jens Stoltenberg denied the creation of the units could be considered provocative by Russia.

"It is completely within all our international obligations and what we are doing is defensive and it is proportionate," he told Reuters in an interview.

The posting of small numbers of NATO soldiers at existing bases in eastern European countries falls short of requests by Poland to have a large NATO base on its soil but creates a symbolic NATO presence and would help the alliance to reinforce the countries rapidly in a crisis.

A NATO diplomat said the units were expected to consist of 40-50 people, split roughly equally between soldiers from the host nation and those from other NATO allies. The units will organize exercises and plan for NATO reinforcement if required.

NATO is working on setting up a rapid reaction force, with some elements able to deploy within two days, that could be sent to eastern Europe in a crisis.

Stoltenberg said the situation on the ground in eastern Ukraine was getting worse and separatists were gaining some ground thanks to "heavy support" they had received from Moscow.

"They have received heavy equipment during a long period of time, hundreds of new pieces of heavy equipment, tanks, rocket systems, artillery and additionally we have seen Russian forces in eastern Ukraine," he said.

"This is in violation of the Minsk ... ceasefire agreements and it undermines the efforts of creating a negotiated peaceful solution based on the Minsk agreements," he said, referring to a peace deal reached in September in the Belarus capital.

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Russia denies having soldiers in eastern Ukraine.

Stoltenberg said NATO would remain the "strongest military alliance in the world," but Russia's spending, including on nuclear weapons, showed Moscow was determined to improve its capabilities.

"What we see from Russia when it comes to nuclear weapons just fits into this broader picture of Russia modernizing and investing heavily in defense, in many different kinds of capabilities, and also having more snap exercises and then also the willingness to use force," he said.

(Editing by Peter Graff)