A fresh round of talks between the US and Russia on air strikes over Syria could take place as soon as this weekend, the Pentagon has said.
The two countries are thrashing out flight procedures to ensure that there are no collisions or other incidents as they conduct bombing operations.
Pentagon press secretary Peter Cook told reporters travelling back from Europe with defence secretary Ash Carter that the Russians sent a formal reply to proposed safety guidelines and officials were currently reviewing it.
The response follows a series of Russian aircraft incidents that have raised concerns about the potential for mishaps, as the US-led coalition continues to launch daily air strikes against Islamic State (IS) militants in Syria.
Russian planes crossed into Turkish airspace last weekend, triggering an immediate protest from Turkey and a vow from Nato that it would defend its member nation. A Russian aircraft also came within a few miles of an American drone.
This week Moscow fired cruise missiles into Syria from ships in the Caspian Sea and the US said four of them went awry and crashed in Iran.
The Pentagon wants an agreement on safety precautions in order to make sure there are no conflicts, misunderstandings or other problems as the coalition and the Russians fly over Syria.
The US side has proposed a number of safety measures, including using specific international radio frequencies for distress calls by military pilots flying in Syrian airspace.
Defence Department officials say there have been no incidents or conflicts with the Russians in the last day or so. US pilots know where the Russian aircraft are and fliers from both sides can see each other, they said.
US and Russian officials spoke once by video conference about the flight issues late last week, before the Russian incursion into the airspace of Turkey, which borders Syria. The US had been waiting since then for a response.
America has warned publicly that there would be no other meetings with Russia until Moscow responded with a written response to the draft, with any suggested changes or edits.
Mr Carter also made it clear that the US would limit any discussions with Russia to basic, technical talks about aircraft safety, saying: “That’s it.”
He has also ruled out any broader co-operation or co-ordination with Russia as long as Moscow continues to strike targets other than the IS militants in Syria.
The Russians have complained about the narrow scope of the US talks and Russian deputy defence minister Anatoly Antonov said Moscow wants broad discussions on international co-operation with the coalition fighting IS.
Russia says the air strikes it began last week are directed against IS, as well as al Qaida’s Syrian affiliates. But the US and other allies say at least some of the strikes appear to have hit Western-backed rebel factions fighting government troops, with the real goal of protecting Syrian president Bashar Assad.