The United Nations on Thursday urged South American states to remain open to people fleeing the crisis in Venezuela, after Ecuador and Peru announced tightened border control measures.
"We recognise the growing challenges associated with the large scale arrival of Venezuelans," UN refugee agency chief Filippo Grandi said in a statement issued jointly with the International Organization for Migration.
"It remains critical that any new measures continue to allow those in need of international protection to access safety and seek asylum," Grandi added.
Colombia on Wednesday said it wanted a special UN envoy and a "multilateral emergency fund" to help manage the mass exodus triggered by Venezuela's economic crisis.
More than a million people have entered Colombia in the last 16 months alone as President Nicolas Maduro's Venezuela grapples with a four-year recession and hyperinflation.
Colombia has granted 800,000 of them temporary residence, but many want to travel onwards to Peru, Chile or even Argentina.
Last week Peru and Ecuador said those without passports will be barred from entering, affecting hundreds of thousands of Venezuelans.
"Of particular concern are the most vulnerable, such as adolescent boys and girls, women, people trying to reunite with their families and unaccompanied and separated children who are unlikely to be able to meet documentation requirements," UNHCR and IOM said.
The UN agencies warned that the new passports requirements will expose people to "further risk of exploitation, trafficking and violence."
According to the UN, 1.6 million Venezuelans have fled the country since 2015.