NATO defense minister agreed on Thursday to expand the alliance’s presence in Eastern Europe in an apparent effort to dissuade Russia from testing its resolve outside Ukraine.

New command and control units are to be set up in Bulgaria, Estonia, Latvia, Lithuania, Poland and Romania. The six countries joined NATO between 1999 and 2004 in the wake of the collapse of the Soviet Union to which they were subordinated during the Cold War.

“If a crisis arises, they will ensure that national and NATO forces from across the alliance are able to act as one from the start,” said NATO’s secretary general, Jens Stoltenberg. “They will make rapid deployment easier.”

The six units will include American troops who are to underpin a new high-readiness or “spearhead” force of 5,000 troops that could instantly react to threats on NATO’s frontier.

The United Kingdom is to provide up to a thousand soldiers.

Although Stoltenberg said there was no “immediate threat” of Russian aggression, he described the alliance’s former Cold War rival’s recent actions in Ukraine as “part of a disturbing pattern of destabilizing Russian behaviour in its neighborhood.”

Poland complained late last year that there was “unprecedented” Russian military activity in the Baltic Sea region and said the country seemed to be testing NATO.

Fighter planes from NATO countries repeatedly intercepted Russian strategic bombers and planes approaching alliance airspace through last year as tensions over the standoff in Ukraine mounted.

In October, Sweden, a non-NATO state, scrambled its naval forces in search of a suspected Russian submarine in its waters.

Russia has played the cat-and-mouse game with the Western military alliance since it occupied and annexed the Crimean Peninsula from Ukraine in March. The incident, which came after a row with the European Union over the bloc’s improving relations with the former Soviet republic and the overthrow of a relatively pro-Russian president in Kiev, alarmed especially Eastern European nations about Russia’s intentions.

Poland, which has the eight largest army in NATO, responded by moving the bulk of its armed forces to its eastern border under a three-year modernization plan.

The alliance deployed surveillance planes for constant patrols over Polish and Romanian territory in order to monitor Russian troop movements in Ukraine.

It also increased the number of jets patrolling Baltic airspace, raised troop levels in Bulgaria, Poland and Romania and held military drills in the east.

Separately, the three Baltic states, Denmark, the Netherlands, Norway and the United Kingdom made plans to form a joint expeditionary force by 2018. The same countries plus Finland and Sweden agreed in November to improve intelligence sharing and joint air force training.

The United States, which account for more than 70 percent of NATO defense spending, announced measures of their own, including increased training activities and the deployment of “rotational” forces in the Baltics and Poland. Fifty American armored fighting vehicles and battle tanks remain prepositioned in those countries with another one hundred armored vehicles to be deployed across Eastern Europe this year.

Previously, the United States shied away from permanently deploying forces in the new NATO states east of Germany for fear of aggravating Russia’s security concerns.

Russia’s invasion of Ukraine at least partially came about as a result of this insecurity complex. The country sees NATO’s eastward expansion as a threat to its ability to defend itself.

Alexander Grushko, the Russian ambassador to NATO, said on Thursday the new spearhead force of 5,000 increases the risk of a “military confrontation” in the Baltic region.