Vladimir Putin condemned the presence of foreign troops in Syria as the Russian president held unannounced talks in Moscow with his counterpart, Bashar Al Assad.
Moscow has been a key ally of the Assad regime throughout the Syrian conflict that erupted in 2011 with the repression of anti-government protests.
Russia's military intervention in 2015 helped turn the tide of the war in the regime's favour and Moscow maintains military bases in the country.
“The main problem, in my view, is that foreign armed forces remain in certain regions of the country without the approval of the United Nations and without your permission,” Mr Putin told Mr Al Assad during their meeting, according to a Kremlin statement.
Mr Putin hailed Russia and Syria's “joint efforts” in the conflict, saying their militaries had returned the vast majority of territory to government control.
“Terrorists have suffered very serious significant damage, and the Syrian government, headed by you, controls 90 per cent of the country's territory,” Mr Putin told Mr Al Assad, the Kremlin said.
The Syrian leader told Mr Putin it was a “great honour” to meet him in Moscow and praised the “significant results” of their two militaries, including work on “the return of refugees who were forced to leave their homes and leave their homeland.”
The Syrian presidency said the two leaders were joined in their talks by Syria's Foreign Minister Faisal Al Meqdad and Russia's Defence Minister Sergei Shoigu.
The two sides discussed the importance of reaching political agreements “between Syrians and without any foreign interference.”
Mr Putin and Mr Al Assad had also discussed “economic co-operation,” the statement said, without elaborating.
Russia's political and military support for Syria has been a particular sticking point in Moscow's relations with the West, which has imposed sanctions on Moscow for bolstering the Assad regime.