WASHINGTON -- U.S. President Joe Biden and his Turkish counterpart, Recep Tayyip Erdogan, agreed earlier this week that Turkey would play a leading role in securing Kabul airport, but the two sides remain divided over the issue of S-400 air defence systems, National Security Adviser Jake Sullivan said on Thursday.

Biden on Monday had his first face-to-face meeting with Erdogan after taking office on the sidelines of the NATO summit.

Sullivan told reporters in a phone briefing that the two leaders had a detailed discussion of a potential Turkish mission to protect the airport following the withdrawal of NATO forces from Afghanistan.

"The clear commitment from the leaders was established that Turkey would play a lead role in securing Hamid Karzai International Airport, and we are now working through how to execute against that," he added.

Sullivan said Biden committed to providing the support that Turkey needed to fulfil that task.

The two leaders made no progress on the issue of Turkey's acquisition of Russian S-400 air defence system, Sullivan noted.

"On the S-400, they discussed it. There was not a resolution of the issue," he said. "There was a commitment to continue the dialogue on the S-400, and the two teams will be following up on that coming out of the meeting."

Russia and Turkey finalized the S-400 air defence system deal worth about 2.5 billion U.S. dollars in 2017, and the delivery was completed in 2019. Turkey is the first NATO member to purchase such a system from Russia.

In response, the United States suspended Turkey's involvement in the F-35 fighter jet program and imposed a series of sanctions against Ankara.