The United States has expressed concern about reports that Russia is moving more military materiel into Syria.
"We think it would be negative for Russia to move additional military equipment or personnel into Syria," Ben Rhodes, deputy national security adviser to President Barack Obama, said on April 21. "We believe that our efforts are best focused on supporting the diplomatic process."
He was speaking in Saudi Arabia, where Obama was attending a summit with Gulf Arab leaders to discuss regional security issues.
Earlier the same day in Ankara, NATO Secretary-General Jens Stoltenberg said Russia has kept a "considerable" military presence in Syria in support of President Bashar al-Assad’s regime.
Stoltenberg made the comments at a joint press conference with Turkish Foreign Minister Mevlut Cavusoglu in Ankara.
On March 14, President Vladimir Putin ordered the "main part" of Russian forces to pull out of Syria, less than six months after the beginning of an air campaign.
Stoltenberg also said the number of migrants crossing the Aegean Sea from Turkey into Europe was "now going significantly down."
Turkey and the EU last month sealed an accord that aims to end the flow of migrants, most fleeing war and poverty in the Middle East, after more than a million reached Europe last year.
NATO's naval back-up in the Aegean Sea has helped to stop people smugglers and reduced the flows.