Russia and US planes 'test safety protocols' in Syria
One Russian and one American plane have carried out a "planned communications test" in the skies over Syria, US officials say.
The Pentagon said the test lasted around three minutes and was designed to "validate the safety protocols" agreed last month.
Last month both countries' planes entered the same "battle space" and came within miles of each other.
After this a deal was signed to avoid clashes between the two air forces.
The test was conducted in "south central Syria" and "assured that the first time this mode of communication was used would not be during an unplanned encounter," a Pentagon statement said.
A senior Russian military official said the test was designed to "to train crews and ground services for incidents of dangerous proximity of aircraft".
In September, Russia started carrying out air strikes against rebels in Syria, after Damascus suffered a string of defeats at the hands of both rebel forces and the Islamic State (IS) group.
Earlier on Tuesday the Russian foreign ministry said it was not crucial for Syria's President Bashar al-Assad to stay in power, saying it was up to the Syrian people to decide.
However, a spokeswoman for the ministry said that this did not represent a change of position.
When asked if saving the Syrian leader was a matter of principle for Russia, spokeswoman Maria Zakharova said: "Absolutely not, we never said that."
Russia is supporting the Syrian government with air strikes on rebels.
Russia is seen as one of Mr Assad's strongest backers. His future is seen as a key sticking point between those backing rival sides in the conflict.
The US has said Mr Assad can have no part in Syria's political future.
Earlier on Tuesday, Russian Deputy Foreign Minister Mikhail Bogdanov said that Moscow was aiming to host a round of talks between Syrian government officials and members of the country's opposition in Moscow next week.
Last week world powers - including key Assad ally Iran for the first time - met in Vienna and agreed to renew efforts to end the conflict.
The ministers agreed to ask the United Nations to start a process that could lead to a ceasefire and new elections. New talks are due in two weeks.
Why is there a war in Syria?
Anti-government protests developed into a civil war that, four years on, has ground to a stalemate, with the Assad government, Islamic State, an array of Syrian rebels and Kurdish fighters all holding territory.
Who is fighting whom?
Government forces concentrated in Damascus and the centre and west of Syria are fighting the jihadists of Islamic State and al-Nusra Front, as well as less numerous so-called "moderate" rebel groups, who are strongest in the north and east. These groups are also battling each other.
What's the human cost?
More than 250,000 Syrians have been killed and a million injured. Some 11 million others have been forced from their homes, of whom four million have fled abroad - including growing numbers who are making the dangerous journey to Europe.
How has the world reacted?
Iran, Russia and Lebanon's Hezbollah movement are propping up the Alawite-led Assad government, while Turkey, Saudi Arabia and Qatar back the more moderate Sunni-dominated opposition, along with the US, UK and France. Hezbollah and Iran are believed to have troops and officers on the ground, while a Western-led coalition and Russia are carrying out air strikes.