Nato's foreign ministers have ordered an end to civilian and military co-operation with Russia and told their generals and admirals to quickly devise ways to better protect alliance members that feel threatened by Moscow.
The 28-member alliance, the keystone of US and European security, was reacting to its most serious crisis in years after Russia annexed Ukraine's Crimean peninsula.
US secretary of state John Kerry and the other ministers, meeting behind closed doors at Nato headquarters in Brussels, unanimously agreed on a number of measures, including possible redeployment of military assets in eastern Nato nations like Poland and the Baltic states.
As the two-day meeting began, the Nato secretary general downplayed reports of a Russian troop pullback from border areas with Ukraine. Russia's defence ministry yesterday said one battalion - about 500 troops - had pulled back.
Nato chief Anders Fogh Rasmussen told reporters: "This is not what we have seen. And this massive military build-up can in no way contribute to a de-escalation of the situation - a de-escalation that we all want to see - so I continue to urge Russia to pull back its troops, live up to its international obligation and engage in a constructive dialogue with Ukraine."
German chancellor Angela Merkel, speaking to reporters in Berlin earlier, echoed those comments: "(Even if some troops left) it's certainly not the final step. The troop concentration on the Ukrainian border is very high."
An estimated 35,000 to 40,000 Russian troops equipped with tanks, other armoured vehicles and fixed and rotary wing aircraft remain near the border with Ukraine, a Nato military official said.
The official described the Russian build-up as "a complete combat force" that was highly threatening to Ukraine.
Earlier, Romania's president said the US had asked to increase the number of troops and aircraft it has stationed at a Black Sea air base in eastern Romania.
Traian Basescu said the US wants to add up to 600 troops to the 1,000 stationed at the base of Mihail Kogalniceanu and military aircraft "for specific missions".
The base, which can accommodate up to 2,000 troops, is a major hub for US forces and equipment leaving Afghanistan and is a few hundred miles from Crimea.
A civilian Nato official who attended the meeting and briefed reporters afterwards said the steps agreed at the meeting included:
:: The suspension of "all practical civilian and military co-operation" between Nato and Russia. Nato officials said ambassadorial-level contacts will remain open to assure a reliable channel of communication.
:: The possible deployment and reinforcement of military assets in eastern Nato members, such as Poland and the Baltic states, that feel menaced by Moscow's latest actions.
:: A possible increase of readiness levels for the Nato rapid response force.
:: A possible review of Nato's crisis response plans, as well as its military training and exercise schedules.
Nato supreme commander General Phil Breedlove and his subordinates will draw up the proposals within a few weeks and submit them to political leaders for their approval, the official said.
To reassure alliance members closest to Russia and Ukraine, Nato already has stepped up air patrols over the Baltic Sea and surveillance flights over Poland and Romania.
Earlier, German foreign minister Frank-Walter Steinmeier renewed a push for internationally backed direct talks between Russia and Ukraine.
"What will be important in the coming days is getting Russia and Ukraine around a table together," he said at a meeting with his French and Polish counterparts in Weimar, Germany, before heading to Brussels.
Despite annexing Crimea, Vladimir Putin and other Kremlin officials have said that Russia has no intention of invading other areas of Ukraine. Defence minister Sergei Shoigu insisted the Kremlin wants a "political settlement that would take interests and rights of the entire Ukrainian people into account".
Also today, Russia sharply hiked the price for natural gas to Ukraine and threatened to reclaim billions in previous discounts, raising the heat on Ukraine's cash-strapped government.
Alexei Miller, the head of Russia's state-controlled Gazprom natural gas giant, said the company had withdrawn December's discount that put the price of 268.50 dollars per 1,000 cubic meters and set the price at 385.50 dollars for the second quarter.
The move is expected to eventually hit Ukrainian consumers hard. Household gas prices in Ukraine are set to rise 50% from May 1.
The Russian discount was part of a financial lifeline Mr Putin offered Ukraine's previous president, Viktor Yanukovych, after his decision to ditch a pact with the European Union in favour of closer ties with Moscow. The move fuelled three months of protests that sent Mr Yanukovych fleeing to Russia in February.