The EU reinstated a travel ban on President Alexander Lukashenko and froze his assets, and the US imposed stricter financial controls and widened its travel bans on senior officials.
The foreign ministry threatened the EU with retaliation over the sanctions.
Mr Lukashenko was re-elected in a disputed vote in December, and launched a crackdown on dissent afterwards.
More than 600 people were detained, including seven of the election candidates, after protests against what international monitors said was a rigged vote.
On Monday, the EU confirmed it had reinstated a travel ban on Mr Lukashenko, and widened the ban to include 157 of his associates.
The EU had originally banned Belarusian leader in 2006 after the last presidential election, but suspended the ban two years later in an attempt to encourage reforms.
The US state department later said it was toughening its sanctions “to respond to the brutal crackdown by President Lukashenko”.
Spokesman Philip Crowley said in a statement that the “disproportionate use of force and initial detentions of hundreds of demonstrators… oblige the United States and others in the international community to act”.
Reuters news agency reported that the US measures included revoking temporary authorisation for business deals with two subsidiaries of Belneftekhim, the country's largest state-owned petroleum and chemical conglomerate.
In a statement released before the US announcement, Belarus's foreign ministry accused the EU of holding a “biased and selective” interpretation of December's election.
“We are calling on the EU to abandon the confrontation rhetoric and hasty attempts to destroy the constructive essence of our relations,” said the statement.
The statement threatened a “proportional and appropriate” response, but gave no further details.
Mr Lukashenko, who has been in power in the ex-Soviet republic since 1994, is often referred to as Europe's last dictator.