Amid escalating tensions between Russia and western powers, the United States and Britain announced Wednesday the deployment of further troops, weapons and fighter jets in Romania and Poland, in the biggest post-Cold War military development on Russia’s borders.

While Britain will send fighter jets to Romania next year, the U.S. will be stationing troops, tanks and artillery in Poland. At a defense meeting Wednesday in Brussels, Germany, Canada and other members of NATO have also pledged forces to the security of their allies.

Tensions have been rife with Russia sending out warships armed with cruise missiles into the Baltic Sea. The alliance also believes that almost 330,000 Russian troops have been stationed on the country’s western border near Moscow, causing NATO to take precautionary measures by establishing a new 4,000-strong force in the Baltics and Eastern Europe.

“This month alone, Russia has deployed nuclear-capable Iskander missiles to Kaliningrad and suspended a weapons-grade plutonium agreement with the United States,” Reuters quoted NATO Secretary-General Jens Stoltenberg as saying.

The alliance aims at deterring Russia’s expansionist outlook after the annexation of Crimea in 2014. The NATO plans to set up four battle groups — a total of 4,000 troops — backed by a rapid reaction force of 40,000 troops. If the need arises, follow-on forces will also be deployed.

U.S. Secretary of Defense Ash Carter has announced a task force of about 900 soldiers to be sent to eastern Poland and another, equipped with tanks and artillery, to cover Eastern Europe. “It’s a major sign of the U.S. commitment to strengthening deterrence here,” Carter said.

Britain has also reaffirmed its backing of the NATO forces with its decision to send an 800-strong battalion to Estonia and Typhoon fighter aircraft to Romania to patrol the Black Sea. British Defense Secretary Michael Fallon was reported as saying: “Although we are leaving the European Union, we will be doing more to help secure the eastern and southern flanks of NATO.”

Relations between Russia and the West have taken a hit since the annexation of Crimea and imposition of sanctions on Moscow. U.S-Russia brokered ceasefire in Syria has also suffered a major setback and Russian President Vladimir Putin has started suspending certain Russia-U.S. defense deals, signaling at a further worsening of ties.