Senate Foreign Relations Committee leaders are questioning the State Department's decision to take Malaysia and Cuba off a U.S. blacklist of countries that have failed to combat modern-day slavery.

Republican committee chair Sen. Bob Corker and ranking Democrat Sen. Ben Cardin wrote to Secretary of State John Kerry requesting a briefing on the annual Trafficking in Persons report, released Monday.

They say they want to better understand the factual basis for the upgrade of several countries. They say U.S. credibility on the issue relies heavily on the integrity of the report, which ranks the performance of 188 governments around the world in combating human trafficking.

Human rights groups and Democratic lawmakers have accused the department of politicizing the process, particularly in the case of Malaysia, a U.S. trading partner.