The former Norwegian prime minister spoke after defence ministers approved a series of measures, including more than doubling Nato's rapid response force, and Washington announced it would pre-position heavy weapons on the alliance's eastern flank.
Russia has denounced the moves as Cold War-style provocations while upgrading its own armed forces, adding more than 40 new nuclear ballistic missiles this year.
"We do not seek confrontation; we do not want a new arms race," Stoltenberg told a press conference after the first day of a defence ministers' meeting at Nato HQ in Brussels.
He said ministers endorsed plans to increase the NATO Response force from 13,000 to up to 40,000 troops, including a spearhead unit of 5,000 soldiers.
Norway's defence minister Ine Eriksen Søreide said that Russia had no reason to feel threatened by Nato's move to increase its military presence on the country's borders.
"This is a response to the uncertainty Russia is creating by its use of military force in eastern Ukraine," she said. "It would have been quite unnatural if not Nato had not sought to find ways to reassure its allies and simultaneously have defensive plans for how to handle any situation that develops."
In his speech, Stoltenberg took a hard line on Russian actions in Ukraine, saying its March 2014 annexation of Crimea was "an act of aggression."
"Russia continues to send troops, forces and supplies into eastern Ukraine. There is no doubt that Russia is responsible for aggressive actions in Europe," he said at the opening of the meeting.
However he also stressed that the the alliance "continues to strive for more constructive relations with Russia."
In the meeting, US Defence Secretary Ashton Carter said Tuesday that Washington would pre-position heavy weapons in central and eastern Europe to ensure any Nato troops responding to a fresh crisis would hit the ground running.
The US equipment includes some 90 Abrams main battle tanks, among the best in the world, 140 Bradley armoured vehicles and 20 self-propelled howitzers.
Estonia, Lithuania, Latvia, Bulgaria, Romania and Poland will store the equipment which will be moved around the region as required for training and exercises.
Russia charges that such deployments breach the 1997 Founding Act with Nato which bans the permanent stationing of significant forces and equipment in former Warsaw Pact states.
Nato says all such deployments are rotational and temporary. The three Baltic states last month called for a permanent Nato presence to deter Russia.
Nato leaders agreed at a September summit to boost defence spending to the equivalent of 2.0 percent of annual economic output after years of cuts following the end of the Cold War.
Stoltenberg said five countries were already above the target, but overall, NATO's combined defence spending would still fall 1.5 percent this year.
In contrast, "Russia has over many years invested heavily in defence," he added.