In a letter to Secretary of State Hillary Clinton, Republican Senator Marco Rubio echoed other lawmakers' concerns in urging the administration to delay the sale of any items in the $53 million weapons package that could be used against protesters in Bahrain.
But State Department spokeswoman Victoria Nuland insisted the weapons sale "is designed to support the Bahraini military in its external defense function, specifically in hardening the country against potential attack or nefarious activity by countries like Iran."
"And we do have an interest in Bahrain being able to be strong militarily vis-a-vis the regional challenges that they face," Nuland added.
She also stressed that Washington considers human rights concerns before seeking arms sales.
The senator's letter came as pressure mounted in Congress against the proposed sale after Bahraini security forces in mid-March crushed a month-long protest that authorities say left 24 people dead, including four policemen.
The opposition puts the death toll at 31.
US lawmakers expressed dismay in January when it came to light that tear gas used against Egyptian demonstrators against deposed president Hosni Mubarak was found to be "Made in America."
Since then, some have sought tighter restrictions on weapons and non-lethal equipment provided to Middle Eastern governments that could be used against protesters in the "Arab Spring" pro-democracy movement roiling the Middle East and North Africa.
Bahrain, located strategically in the Gulf opposite Iran, houses the headquarters of the US Fifth Fleet.