"In the past days, a number of Allies have made announcements regarding current or upcoming deployments," NATO said in a statement posted on its website.
It added: "Denmark is sending a frigate to the Baltic Sea and is set to deploy four F-16 fighter jets to Lithuania in support of NATO’s long-standing air-policing mission in the region. Spain is sending ships to join NATO naval forces and is considering sending fighter jets to Bulgaria. France has expressed its readiness to send troops to Romania under NATO command. The Netherlands is sending two F-35 fighter aircraft to Bulgaria from April to support NATO’s air-policing activities in the region, and is putting a ship and land-based units on standby for NATO’s Response Force. The United States has also made clear that it is considering increasing its military presence in the eastern part of the Alliance."
NATO Secretary-General Jens Stoltenberg said: “I welcome Allies contributing additional forces to NATO."
He added that NATO would "continue to take all necessary measures to protect and defend all Allies, including by reinforcing the eastern part of the Alliance."
"We will always respond to any deterioration of our security environment, including through strengthening our collective defence,” Stoltenberg was quoted as saying.
The statement on the nato.int website also said that "in response to Russia’s illegal annexation of Crimea in 2014," NATO increased its presence in the eastern part of the alliance, including with four multinational battlegroups in Estonia, Latvia, Lithuania and Poland.
"These units, led by the United Kingdom, Canada, Germany and the United States respectively, are multinational, and combat-ready," NATO said.
"Their presence makes clear that an attack on one Ally will be considered an attack on the whole Alliance," it added.
The statement noted that "there were no NATO forces in the eastern part of the Alliance before 2014."