BRUSSELS (AA) A meeting of the Turkish and US presidents during the G20 summit in Japan could help resolve the issue of Turkey’s purchase of Russian S-400 defense systems, said NATO’s chief on Thursday.

When Turkish President Recep Tayyip Erdogan and US President Donald Trump meet in Japan this week, “hopefully that can bring some progress,” Jens Stoltenberg told reporters following a two-day NATO defense ministers’ meeting in Brussels.

Welcoming dialogue between Turkey and the US on the “difficult” S-400 issue, Stoltenberg said it was not part of the NATO meeting but was discussed on its sidelines.

Tensions between the US and Turkey have escalated in recent months over the Russian S-400 air defense systems purchase.

, which Washington said will jeopardize Turkey’s role in the F-35 fighter jet program and could trigger sanctions.

Following protracted efforts to purchase an air defense system from the US with no success, Ankara decided in 2017 to purchase the Russian S-400s. US officials urged Turkey to buy US Patriot missiles, arguing the Russian system would be incompatible with NATO systems and expose the F-35s to possible Russian subterfuge.

Turkey, however, emphasized the S-400 would not be integrated into NATO systems and would not pose a threat to the alliance. Turkey recently urged the formation of a commission to clarify any technical issues, but the US has failed to respond to this proposal.

On Thursday’s defense ministers’ meeting, Stoltenberg said the allies also discussed developments in Afghanistan. NATO will continue to provide financial support to Afghan security forces through 2024, he said.

Stoltenberg added that they will continue to be present in the country as long as is necessary, in order to prevent Afghanistan from again being a safe haven for international terrorists. He said in another step at the meeting, a new policy on outer space was approved.