Belarus says it is ready to "play its role" as a Russian ally if the US overrides Moscow's objections and creates new missile bases in Europe.
Mr Lukashenko (right) is seen as one of Mr Putin's firmest allies
Russia has warned it may place missiles in Belarus to counter US plans for bases in Poland and the Czech Republic.
Belarussian leader Alexander Lukashenko praised ties with Russia as he hosted President Vladimir Putin in Minsk.
But he dismissed speculation that his country, a former Soviet state, might enter a formal union with Russia.
"I was surprised your visit prompted a stir in the West," Mr Lukashenko said to Mr Putin during the Russian leader's trip to the Belarussian capital.
"There's no subtext here. We're friendly allied states and I would be surprised if you didn't visit."
Analysts have recently suggested that a union between Minsk and Moscow could enable President Putin to retain political influence in Russia after he relinquishes the presidency in March.
Mr Putin could, in theory, become leader of the new entity created by such a union.
Mr Putin has also been tipped as a candidate for prime minister in Russia after his presidential term ends.
His exact plans are not yet known, though it is clear he intends to capitalise on his popularity and continue playing a central role in politics.
Mr Lukashenko welcomed Mr Putin at a ceremony in Minsk.
The Belarus armed forces keep many Soviet-era traditions alive
"Belarus is ready to play its role in the issues of the planned deployment in Europe of US missile defence systems," Mr Lukashenko said.
He did not specify what this role would be.
Last month, a senior Russian general said his country may place missiles in Belarus to counter US plans for bases in central Europe.
The US says it plans to build a missile defence system by building bases in the Czech Republic and Poland that could help intercept missiles fired from countries such as Iran.
Russia has however dismissed the alleged threat from Iran and said the US is targeting its territory.
Moscow has repeatedly voiced alarm at Nato's eastward expansion plans, encompassing nations that were once in the Soviet Union's sphere of influence.
Belarus is largely regarded as one of Russia's staunchest allies among the ex-Soviet states.
However, ties between the two countries were recently strained over Russian efforts to raise the price of fuel it supplies to Belarus.