The bloc's chief says it will be the biggest reinforcement of its collective defence since the end of the Cold War.
Meanwhile, US Secretary of State John Kerry is to hold talks in Kiev as the US considers whether to send weapons to help Ukraine fight pro-Russian rebels.
The US has so far only provided "non-lethal" assistance to Ukraine.
On Wednesday Ashton Carter, the White House's choice for defence secretary, said he was "inclined" to start supplying arms.
Rapid reaction NATO defence ministers meeting in Brussels on Thursday will seek to reassure the alliance's member nations in Eastern Europe by boosting its forces there.
A rapid reaction force of up to 5,000 is expected to be announced, with its lead units able to deploy at two days' notice.
NATO chief Jens Stoltenberg has said the bloc faces a "fundamental change" to its security environment because of Russian aggression.
Speaking as ministers arrived in Brussels, he said:
"This is something we do as a response to the aggressive actions we have seen from Russia, violating international law and annexing Crimea," he said.
"I very much underline that this is something we do because we have to adapt our forces when we see that the world is changing."
NATO will also reveal plans for a network of small command centres in Estonia, Lithuania, Latvia, Poland, Romania, and Bulgaria.
The move is being seen as a potential deterrent against any Russian threat to the Baltic states or other bloc members should the crisis in Ukraine spin out of control.
Russia denies accusations by Ukraine and the West that it is arming rebels in eastern Ukraine and sending regular troops across the border.
NATO sees its actions as entirely defensive - but this is not the way they will be seen in Moscow.
Indeed their fundamentally different perceptions of the Ukraine crisis is what is driving the wider rift between Russia and the West.
NATO's wider actions - it also plans to open a training centre in Georgia and support for the reform of Ukraine's military - all ring alarm bells in Moscow.
Tensions could get worse still if the US or other NATO allies move to arm the Ukrainian military.
This is not a NATO issue as such but something for national governments, and everyone is watching the course of the evolving debate within the Obama administration.
Later on Thursday, Mr Kerry will meet Ukrainian President Petro Poroshenko and Prime Minister Arseniy Yatsenyuk in Ukraine's capital.
The issue of weapons deliveries to Ukraine - and other avenues of US assistance - is expected to be one of the main items on the agenda.
On Wednesday Mr Carter, who previously served as deputy secretary of defence, appeared in front of the Senate Armed Service committee in Washington for questioning ahead of a full Senate confirmation vote on his nomination as Secretary of Defense.
He spoke positively on the possibility of moving arms supplies to the Ukrainians.
"I'm very much inclined in that direction, mister chairman, because I think we need to support the Ukrainians in defending themselves," he said when asked by Senator John McCain if he supported delivering "defensive weapons" to Ukraine.
"The nature of those arms, I can't say right now," he added.
The White House has previously expressed fears that sending in weapons could trigger a tense confrontation with Russia and escalate the conflict.
But US President Barack Obama is now said to be reconsidering his position, accusing Russia of escalating the conflict in Ukraine's eastern Donetsk and Luhansk regions.
Fighting has intensified in recent weeks, leaving a September ceasefire in tatters.
Earlier this week, a group of former senior US officials and officers urged a major increase in military assistance, including providing light-armour missiles designed to take out tanks and armoured vehicles.
And on Tuesday, a group of US senators called on President Obama and NATO "to rapidly increase military assistance to Ukraine to defend its sovereign borders against escalating Russian aggression".
Fighting in eastern Ukraine began last April, when separatists seized government buildings after Russia annexed Ukraine's southern Crimea peninsula.
More than 5,300 people have been killed in the conflict.
Source: BBC News Europe