President Joe Biden signed an order on Friday limiting US refugee admissions this year to the historically low 15,000 cap set under his predecessor Donald Trump, shelving a plan to raise it to 62,500 and drawing the ire of refugee advocates and some Democratic lawmakers. 

But as criticism mounted, the White House issued a statement saying Biden would set a "final, increased refugee cap" for the remainder of this fiscal year by May 15.

Biden's order to limit admissions to 15,000 was a blow to advocacy groups that wanted the Democratic president to move swiftly to reverse the refugee policies of the Republican Trump, who had set the figure as a way to limit immigration.

The program for admitting refugees is distinct from the asylum system for migrants. Refugees must be vetted while still overseas and cleared for entry to the US, unlike migrants who arrive at a US border and then request asylum.

Biden pledged in February to increase the number of refugees admitted in the next fiscal year to 125,000.

Democratic Representative Alexandria Ocasio-Cortez wrote on Twitter that the cap was "completely and utterly unacceptable."

"Biden promised to welcome immigrants, and people voted for him based on that promise," Ocasio-Cortez wrote. 

Critics say Biden is reading carefully on the issue as migrant crisis deepens at the Mexico-US border.