The North Atlantic Treaty Organization, or NATO, on Monday, said that it was keeping forces on standby in response to Russia’s troop build-up along the Ukraine border, Reuters reported.

NATO added that it is reinforcing the eastern European region with more ships and fighter jets because of the emerging situation.

NATO Secretary General Jens Stoltenberg said that the alliance will “always respond to any deterioration of our security environment, including through strengthening our collective defence”. He added that an enhanced presence in eastern Europe could include the deployment of battle groups.

Stoltenberg, leaders of European countries and representatives of the European Union on Monday held a meeting with United States President Joe Biden to discuss the situation at the Russia-Ukraine border. “We agree that any further aggression by Russia against Ukraine will have severe costs,” the NATO general secretary said.

Meanwhile, Pentagon Press Secretary John Kirby on Monday said that the country has put about 8,500 troops on alert to be deployed in Europe if the situation so arises. Kirby said that Biden has made it clear that “the United States will act firmly in defence of its national interests in response to actions by Russia that harm us, our allies or partners”.

Biden on Monday said that he had “a very, very, very good meeting” with European leaders, and said that there was total unanimity, AP reported.

Meanwhile, Ukraine’s President Volodymyr Zelensky called on the European Union to stay united to deal with a possible invasion of his country by Russia, AFP reported.

Ukraine and Russia have been engaged in a conflict since 2014 when Moscow annexed Crimea and backed separatist forces in Kyiv’s eastern regions of Donetsk and Luhansk. The tensions between the countries have intensified after Russia, last year, gathered 1 lakh troops near the Ukrainian border.

Last week, a series of talks held in Europe had failed to ease tensions between the two countries.

Russia has denied any plans to invade Ukraine. On Monday, Kremlin spokesperson Dmitry Peskov accused the West of engaging in “hysteria”, according to Reuters.

“This is not happening because of what we, Russia, are doing,” he said. “This is all happening because of what NATO and the US are doing and due to the information they are spreading.”