US Vice President Joe Biden has reassured Europe's Baltic countries of Washington's commitment to protect its NATO alliance during a visit to Latvia.

"The concern that the United States is somehow disengaging or leaving is simply not true. As a matter of fact, our presence will be enhanced and the fact is that there will be more NATO partners on the Russian border. There will be more than a single tripwire. We are absolutely, thoroughly, 100 percent committed to our NATO obligations including and especially Article 5," Biden said.

Biden's visit comes amid tension between the Baltics states and Russia. Earlier, the Republican presidential nominee, Donald Trump suggested he might abandon NATO's pledge to defend all alliance members if elected. Biden said Trump's comment could not be taken seriously.