US Secretary of State Condoleezza Rice has described Belarus as the "last true dictatorship" in central Europe and has called for the country to change.
Rice has described Belarus as an 'outpost of tyranny'
Speaking in neighbouring Lithuania, Ms Rice said she thought it was no bad thing for ex-Soviet republics to "throw off the yoke of tyranny".
Belarus has responded angrily to the comments, accusing Ms Rice of meddling in the country's affairs.
And Russia said reform should not be "imposed from outside".
Russian Foreign Minister Sergei Lavrov said: "We would not of course be advocating what some people call regime changes anywhere.
"We think the democratic process, the process of reform cannot be imposed from outside."
Ms Rice is in Vilnius for Nato meetings having met President Putin in Moscow.
The secretary of state told a press conference she believed it was "time for change to come to Belarus".
But Belarus Foreign Minister Sergei Martynov told Interfax news agency that his country's future would be "determined by the nation's people and not US Secretary of State Condoleezza Rice".
Ms Rice has previously listed Belarus among six "outposts of tyranny" in the world, along with Iran, Cuba, North Korea, Burma and Zimbabwe.
In his state of the nation address on Tuesday, President Lukashenko warned he would take firm action against countries and groups he suspected of trying to undermine him.
He has repeatedly accused the west of fomenting unrest across the former Soviet Union.
The Nato meeting is the first to be held on ex-Soviet Union territory.
Nato missions in Afghanistan, Iraq and Kosovo - as well as possible Nato involvement in the Middle East - will be on the summit's agenda.
Ms Rice arrived from Moscow where she accused Mr Putin of wielding too much power and called for a free and independent media in Russia.