Russia and the US have signed a deal that they hope will avoid clashes between their air forces in the skies over Syria, officials from both countries said.
Russia began air strikes in Syria on September 30, saying it was targeting Islamic State militants.
Last week, the US said both countries' planes "entered the same battle space" and came within miles of each other.
Officials had been seeking an agreement since late September.
Pentagon spokesman Peter Cook said the text of the deal would remain secret at Moscow's request, but that it laid out means for both sides to communicate and establish a hotline on the ground.
The two countries would not, however, share intelligence on their targets, he said.
The deal also ensured aircraft would stay a "safe" distance from each other, but he would not confirm if specific distances were agreed.
Last week, the Pentagon said Russian and US combat aircraft were in visual contact with each other, 15 to 30 kilometres apart.
Russia's deputy defence minister Anatoly Antonov said the memorandum "contains a number of rules and restrictions aimed at preventing incidents between American and Russian planes".