A reinvigorated NATO flexed old Cold War muscles Tuesday as the Atlantic alliance’s chief recommitted to defending Eastern European and Baltic nations rattled by Russia’s military moves and its annexation of Crimea.

At the opening of a two-day meeting of NATO foreign ministers, Secretary General Anders Fogh Rasmussen said the alliance has seen no signs of Russian troop withdrawals along the Ukraine border, as Moscow has claimed. A senior U.S. State Department official had called Russia’s promised pullback a “gesture,” but a welcome one.

“We have all challenged the tactics of intimidation,” Secretary of State John F. Kerry said after meeting Tuesday with NATO members and Ukrainian envoys.

The alliance moved to suspend many military and civilian ties with Russia over its military incursion and annexation of Crimea, but it stopped short of ordering new troop deployments of its own, a move that could provoke a larger confrontation.

“NATO has consistently worked for closer cooperation and trust with Russia” for two decades, the alliance ministers said in a statement. “However, Russia has violated international law” and its agreements with NATO, they said. “It has gravely breached the trust upon which our cooperation must be based.”

Although NATO has largely refrained from posting troops in nations at Russia’s front door, Rasmussen did not rule out doing so in the future. NATO could establish permanent bases in front-line allied nations, alliance officials said this week.

Ukraine is not a member of the alliance but cooperates with it, to Russia’s frequent dismay. Ukraine’s foreign minister reiterated Tuesday that his nation is not seeking NATO membership but is exploring greater cooperation.

NATO foreign ministers agreed Tuesday to intensify the alliance’s partnership with Ukraine and provide additional assets to Eastern European partners.

“Russia’s aggression against Ukraine challenges our vision of a Europe whole, free and at peace,” Rasmussen said. “We are now considering all options to enhance our collective defense, including an update and further development of defense plans, enhanced exercises and also appropriate deployment.”

The United States has joined Black Sea naval exercises, and NATO members have increased air patrols over the Baltic states and employed AWACS surveillance planes over Poland and Romania.

Eastern European leaders have expressed unhappiness with the pace at which NATO has sought to bulk up its presence on the front lines with Russia. Polish Prime Minister Donald Tusk said the results have been “unsatisfactory.”

“We are gaining something step by step, but the pace of NATO increasing its military presence for sure could be faster,” he said.

In Washington, the House approved an aid package Tuesday for Ukraine, whose Western-oriented interim leaders have committed to economic and political reform and elections in May. The Senate voted on the measure last week.

The 28-member alliance’s regular spring meeting of foreign ministers was energized by Russia’s move to invade and then annex Crimea from Ukraine, and to deploy as many as 40,000 troops along the border with Ukraine.

NATO, formed as a U.S.-backed bulwark against the Soviet Union, has expanded in the past 15 years to include many former Soviet satellite states, often over Russian complaints. NATO sometimes invites Russia to attend sessions, but that was not the case this time.

“We will make clear that Russia’s actions are unacceptable. We will take decisions on which cooperation with Russia is still appropriate,” Rasmussen said. “Russia has undermined the principles on which our partnership is built and has breached its own international commitments. So we cannot go on doing business as usual.”

“Clearly we have a NATO that is now in the business, in the context of events in and around Ukraine, of rebalancing its mission,” a senior U.S. official said.

NATO has focused for two decades on joint missions, such as in Kosovo and Afghanistan. Without the organizing principle of collective defense against the Soviet Union, however, the alliance has sometimes seemed adrift. The United States has complained that it is funding a disproportionate share of NATO missions while European nations cut their defense budgets.

Those missions are still important, the U.S. official said, but after the Russian show of force, “the alliance is now very much focused once again on its core and founding mission, which is defense of allied territory.” The official spoke on the condition of anonymity to describe U.S. objectives ahead of the meetings.

Rasmussen said Tuesday that allied intelligence does not show that Russia is scaling back what he called a “massive military buildup.”

The Russian Defense Ministry announced Monday that a motorized infantry battalion would return to its home base after deployment along the border with eastern Ukraine. A battalion can range from a few hundred to about 1,500 soldiers.

Russia has maintained that it is conducting legal, routine military training and has no intention of extending its reach beyond Crimea. But Ukrainian, U.S. and European officials have said that massing troops is a form of intimidation against the government in Kiev and warned that an incursion could occur at any moment.

The United States and European allies consider Russia’s annexation of Crimea illegal and insist that it will never be recognized. Washington and the European Union have approved two rounds of relatively mild sanctions and are considering more far-reaching penalties on Russian oil and gas industries.

President Obama signed an executive order last month authorizing further sanctions if Russian forces enter Ukraine beyond Crimea.

On Tuesday, a 44 percent price increase on much of the natural gas sold to Ukraine’s Naftogaz by Russia’s Gazprom monopoly went into effect. That brought the price of natural gas sold to Ukraine in line with, or slightly above, the prevailing prices Gazprom charges other European customers.

“Ukraine will be unable to pay this higher rate without Western financing, which will heighten the prospect that Gazprom cuts some of Ukraine’s gas shipments,” said analysts at the Eurasia Group. Last month, Gazprom chief executive Alexey Miller said Naftogaz was already $1.5 billion in arrears on its payments. Although warm weather and ample gas in storage will moderate the immediate effect on Ukraine’s economy, the higher gas price will be an extra burden next winter.

Former Ukrainian president Viktor Yanukovych had negotiated a discount late last year; the new rate is in line with prices Ukraine had been charged earlier.

A separate discount had been negotiated in 2010 as part of an agreement signed in Kharkiv, Ukraine, to extend Russia’s lease of the Crimean port of Sevastopol for its Black Sea Fleet. Although Russian Prime Minister Dmitry Medvedev said that discount was no longer necessary, it is not subject to a quarterly review like the other discount. Eurasia Group analyst Emily Stromquist said legislation that would cancel the Crimean port lease — and the Kharkiv agreement — is in Russia’s upper house for final approval, after which Moscow could revoke that discount, as well.

Steven Mufson in Washington contributed to this report.