TALLINN, Estonia — U.S. Vice President Mike Pence on Monday strongly pledged America’s commitment to protecting NATO allies against attacks, including the Baltic states, which have anxiously watched a growing Russian military presence in the region.

“Under President Donald Trump, the United States stands firmly behind our Article 5 pledge of mutual defense — an attack on one of us is an attack on us all,” Pence told reporters after meeting with the presidents of Estonia, Latvia and Lithuania in the Estonian capital of Tallinn.

Mutual defense is a vital issue for the three small former Soviet states that border Russia, which were all occupied for nearly five decades by Soviet troops before regaining their independence in 1991 after the collapse of the Soviet Union.

Noting that Trump “knows security is the foundation of our prosperity,” Pence said America and the Baltic countries would seek new ways to increase prosperity by increasing two-way trade that currently amounts to $3.5 billion and increasing mutual investments.

Earlier, he met Estonian President Kersti Kaljulaid, the president of Latvia, Raimonds Vejonis, and Lithuanian President Dalia Grybauskaite.

Estonia currently holds the rotating presidency of the 28-nation European Union.

Pence is also scheduled to meet NATO troops from Britain, France and the United States that are stationed in Estonia. The alliance has deployed some 4,000 troops and military hardware in the three Baltic states and Poland to counter Russia’s presence in the Baltic Sea region.

Pence is in Estonia on the first leg of a European tour that also takes him to Georgia and Montenegro, two other regions facing strong pressure from Russia.