WASHINGTON (Reuters) - A U.S. Senate committee passed a spending bill on Thursday including a provision to block Turkey’s purchase of Lockheed Martin F-35 Joint Strike Fighter jets unless it drops a plan to buy S-400 missile defense systems from Russia.

The measure is an amendment to the Fiscal Year 2019 State, Foreign Operations and Related Programs Appropriations Act.

If it became law, the measure would block the transfer of F-35s to Turkey until the Secretary of State certified that Turkey is not purchasing and will not accept deliveries of the Russian systems.

Relations between Ankara and Washington have been strained over a host of issues in recent months, including U.S. policy in Syria and a number of legal cases against Turkish and U.S. nationals being held in the two countries.

The amendment is the latest effort by members of Congress to influence NATO ally Turkey not to go ahead with its plan to buy the Russian system. A similar measure was included in a defense policy bill making its way through the legislature.

The move to buy S-400s, which are incompatible with NATO systems, has unnerved NATO member countries, which are already wary of Moscow’s military presence in the Middle East.

“Turkey’s acquisition of both systems would allow the Russians to more easily evaluate the capabilities of the F-35 and detect and exploit its vulnerabilities. That is unacceptable,” Democratic Senator Chris Van Hollen, who backed the amendment, said in a statement.

The spending bill is several steps from becoming law. It must still be approved by the full Senate and House of Representatives, and signed by President Donald Trump.

Separately, government and Lockheed Martin officials held a ceremony on Thursday to mark the “roll out” of the first F-35A Lightning II jet for Turkey. The F-35 typically hosts such a ceremony to recognize every customer’s first aircraft.

That aircraft is headed to Arizona, where F-35 training takes place. Delivery of the jets into Turkey itself is not expected until next year.