As the United States begins to wade into the refugee crisis straining Syria’s neighbors, national security concerns competing against a swift resettlement response are dividing lawmakers along party lines.

Obama administration immigration and refugee officials traveled to Capitol Hill Thursday to smooth security concerns at a congressional hearing as the U.S. begins to open its borders to 10,000 Syrian refugees seeking haven.

Republican Sen. Jeff Sessions of Alabama, chairman of the Senate Judiciary Committee’s immigration subcommittee, expressed grave skepticism regarding the administration’s ability to properly vet the refugees given the potential for document falsification and terrorist connections.

“I don’t believe you can tell us with any certainty that you have the ability to conduct an efficient background check,” Sessions told Homeland Security officials during the Senate Judiciary hearing.

Barbara Strack, chief of the Refugee Affairs Division at USCIS, outlined the department’s extensive screening process for refugee applicants, including a multilayer biographic background check, a thorough interview process, and the use of fingerprints cross-checked across intelligence agencies.

Despite stressing a meticulous application process, Strack said the current approval rate for Syrian refugees runs over 90 percent. The U.S.’s worldwide average approval rate is about 80 percent.

She noted that the number excludes pending cases and predicted that the rate would fall as USCIS assesses more applications.

Sessions pressed Strack on the high acceptance rate, citing concerns of the Islamic State in Iraq and Syria infiltrating operatives among refugees attempting to emigrate into the U.S.

“There are a lot of people who would like to become a refugee to the United States or Europe,” Sessions said. “I’m not doubting your dedication to try to do right, but the ability that you have.”

The hearing coincided with President Barack Obama’s plan to take in 10,000 Syrian refugees during the new fiscal year, which began Thursday.

Last week, U.S. Secretary of State John Kerry announced that the U.S. would escalate its resettlement plan, vowing to accept 85,000 refugees from around the world in 2016, rather than 70,000. That number will grow to 100,000 in 2017.