A Russian and an American plane have conducted a "test of the communications provided" in the skies of Syria, US officials said.
The Pentagon said the test lasted about three minutes, was designed to "validate security protocols" agreed last month.
Last month the two countries aircraft entered the same "battle space" and were a mile apart.
After this, an agreement was signed to prevent clashes between the two air forces.
The test took place in "the south central Syria" and "confident that the first time that the media would not be used during an unplanned meeting," said a Pentagon statement.
A Russian senior military official said the trial was designed to "train crew and ground services for incidents of dangerous proximity of the aircraft."
In September, Russia began carrying out air strikes against rebels in Syria, after Damascus suffered a series of defeats at the hands of both rebels and the State Islamic Group (IS).
Should there be a no-fly zone Syria?
High-stakes bet on Syria
If Assad does not, then who?
When key countries stand
Earlier on Tuesday, the Foreign Ministry of Russia said it was not crucial for Syrian President Bashar al-Assad to stay in power, saying it was up to the Syrian people to decide.
However, a ministry spokesman said this did not represent a change in position.
When asked if saving the Syrian leader was a matter of principle for Russia, spokeswoman Maria Zakharova said: "Of course not, never said that."
Russia supports the Syrian government airstrikes against the rebels.
Russia is seen as one of the strongest supporters of Mr. Assad. Your future is seen as a key point between rival factions supporting the conflict.
The USA. He said Mr. Assad may have a part in the political future of Syria.
Earlier on Tuesday, Russian Deputy Foreign Minister Mikhail Bogdanov said 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, the world powers - including key ally of Assad to Iran for the first time - met in Vienna and agreed to renew efforts to end the conflict.
The ministers agreed to request the United Nations to initiate a process that could lead to a high fire and new elections. Further talks are due in two weeks.
Smoke by the Syrian army after the bombing rises in Khobar, DamascusImage copyrightAP
Why is there a war in Syria?
Anti-government protests turned into a civil war four years later, has ground to a standstill, with the Assad government, Islamic state, a number of Syrian rebels and Kurdish fighters have any territory.
Who is fighting whom?
Government forces were concentrated in Damascus and the central and western Syria are fighting Islamic jihadists State and al-Nusra Front and fewer so-called "moderate" rebel groups, which are stronger in the north and east. These groups are also fighting each other.
What is the human cost?
More than 250,000 Syrians have died and a million wounded. Some 11 million more have been forced from their homes, of which four million have fled abroad - including increasing numbers are making the perilous journey to Europe.
How has the world reacted?
Iran, Russia and the Lebanese Hezbollah movement are supporting the government of Assad Alawite-led, while Turkey, Saudi Arabia and Qatar back to more moderate Sunni opposition, along with the US, UK and France. Hezbollah and Iran are believed to have troops and officers on the ground, while a coalition led western Russia are conducting air strikes.
The Pentagon said the test lasted about three minutes, was designed to "validate security protocols" agreed last month.
Last month the two countries aircraft entered the same "battle space" and were a mile apart.
After this, an agreement was signed to prevent clashes between the two air forces.
The test took place in "the south central Syria" and "confident that the first time that the media would not be used during an unplanned meeting," said a Pentagon statement.
A Russian senior military official said the trial was designed to "train crew and ground services for incidents of dangerous proximity of the aircraft."
In September, Russia began carrying out air strikes against rebels in Syria, after Damascus suffered a series of defeats at the hands of both rebels and the State Islamic Group (IS).
Should there be a no-fly zone Syria?
High-stakes bet on Syria
If Assad does not, then who?
When key countries stand
Earlier on Tuesday, the Foreign Ministry of Russia said it was not crucial for Syrian President Bashar al-Assad to stay in power, saying it was up to the Syrian people to decide.
However, a ministry spokesman said this did not represent a change in position.
When asked if saving the Syrian leader was a matter of principle for Russia, spokeswoman Maria Zakharova said: "Of course not, never said that."
Russia supports the Syrian government airstrikes against the rebels.
Russia is seen as one of the strongest supporters of Mr. Assad. Your future is seen as a key point between rival factions supporting the conflict.
The USA. He said Mr. Assad may have a part in the political future of Syria.
Earlier on Tuesday, Russian Deputy Foreign Minister Mikhail Bogdanov said 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, the world powers - including key ally of Assad to Iran for the first time - met in Vienna and agreed to renew efforts to end the conflict.
The ministers agreed to request the United Nations to initiate a process that could lead to a high fire and new elections. Further talks are due in two weeks.
Smoke by the Syrian army after the bombing rises in Khobar, DamascusImage copyrightAP
Why is there a war in Syria?
Anti-government protests turned into a civil war four years later, has ground to a standstill, with the Assad government, Islamic state, a number of Syrian rebels and Kurdish fighters have any territory.
Who is fighting whom?
Government forces were concentrated in Damascus and the central and western Syria are fighting Islamic jihadists State and al-Nusra Front and fewer so-called "moderate" rebel groups, which are stronger in the north and east. These groups are also fighting each other.
What is the human cost?
More than 250,000 Syrians have died and a million wounded. Some 11 million more have been forced from their homes, of which four million have fled abroad - including increasing numbers are making the perilous journey to Europe.
How has the world reacted?
Iran, Russia and the Lebanese Hezbollah movement are supporting the government of Assad Alawite-led, while Turkey, Saudi Arabia and Qatar back to more moderate Sunni opposition, along with the US, UK and France. Hezbollah and Iran are believed to have troops and officers on the ground, while a coalition led western Russia are conducting air strikes.
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