The United States on Monday slapped sanctions on senior Venezuelan government officials accused of taking bribes to issue passports, the US Treasury said.

The sanctions targeted Gustavo Adolfo Vizcaino Gil, head of the government department in charge of identification documents and migration, as well as his predecessor Juan Carlos Dugarte Padron, the Treasury's Office of Foreign Assets Control said.

It said the scheme to sell passports for thousands of dollars dated back at least to April 2016 when Dugarte was named as head of the government department and continued under Vizcaino, who took over in June 2018.

More than 4.5 million Venezuelans have fled the country, mostly for neighboring Latin American countries, after years of economic crisis.

That outflow is expected to grow to 6.5 million in the coming year, rivalling the refugee crisis from Syria, according to a UN-EU conference in October.

The US Treasury, which calls President Nicolas Maduro's government "the former illegitimate regime," has tightened its sanctions grip on government officials and on the state oil company PDVSA which provides most of the country's resources.