Two key US senators expressed concerns Tuesday about a possible agreement to base a missile-shield radar in Turkey, citing the NATO ally's strained ties with Israel and relations with Iran.

Republican Senators Jon Kyl and Mark Kirk wrote Defense Secretary Leon Panetta and Secretary of State Hillary Clinton seeking reassurances on the possible deal, which was described in a news report last week.

The lawmakers asked for "written assurances" that data collected by a so-called X-band radar "will be made available, in real time" to staunch US ally Israel to be "fully integrated into its battlement management and control."

They also sought a guarantee that "Turkish entities are not engaged, or suspected of engaging" in activities that fall afoul of various US laws aimed at curbing suspected nuclear weapons programs in Iran and Syria and keeping sensitive know-how from North Korea.

And President Barack Obama's administration must also certify that the powerful radar will only be operated by US personnel, and for 24 hours a day, seven days a week, except for maintenance breaks, the senators said.

Kirk and Kyl, the number-two Senate Republican, also questioned whether the reported decision to locate the radar in Turkey would "ensure the best defense of the United States against the Iranian long-range ballistic missile threat."

They cited a US Missile Defense Agency study that found that the South Caucasus to be "the optimum placement" if the system is designed to defend against an eventual Iranian ballistic missile attack.

"The administration's plans for missile defense will require the cooperation of the Congress; the prospects for such cooperation are jeopardized if the Congress is not provided the information it requests," they warned.