The United States said it is concerned about reports that Russia is moving more military equipment and personnel back into Syria, while the top NATO official said the cease-fire still remains the best chance for a peaceful solution to the crisis.
"It would be negative for Russia to move additional military equipment or personnel into Syria," said Ben Rhodes, the U.S. deputy national security advisor to President Barack Obama. "We believe that our efforts are best focused on supporting the diplomatic process."
U.S. defense and intelligence officials said Russia has been steadily increasing its presence around the key northern Syrian city of Aleppo, ahead of a renewed push by forces loyal to Syrian President Bashar al-Assad.
Putin, Obama call
Earlier this week, in a telephone call with Russian President Vladimir Putin, Obama stressed the importance of all of the parties living up to commitments made when the cessation of hostilities was agreed to.
Russia announced in March that it was partially withdrawing its military forces from Syria, however NATO chief Jens Stoltenberg said it "maintains a considerable military presence" in support of Assad's government.
Stoltenberg, who spoke Thursday in Ankara, said that although the nearly two-month-old cease-fire is "under strain," it still represents Syria's best chance at a peace.
The cessation of hostilities between pro-government forces and opposition fighters went into effect at the end of February, and has been widely credited with sharply reducing fighting in Syria.
But both sides have reported numerous violations, including a rise in fighting the past few weeks.
Stoltenberg, who met with Turkish Foreign Affairs Minister Mevlut Cavusoglu, welcomed Turkey's participation in international efforts to defeat the Islamic State militant group, including allowing its Incirlik Airbase to be used by the U.S.-led coalition conducting airstrikes against the militants.
Turkey's warplanes have been taking part in the airstrikes in Turkey, but not Iraq.