United Nations Secretary-General Ban Ki-moon is in Russia, where he has said it is important not to "lose momentum" in talks on setting up a peace conference on Syria next month.
Mr. Ban told reporters Friday that momentum gained by recent talks between U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov should be built upon, rather than wasted. He also said a U.N. team is ready any time to investigate claims that the government of Syrian President Bashar al-Assad is using chemical weapons against rebels.
Mr. Ban is in the Black Sea resort town of Sochi to meet with Russian President Vladimir Putin for talks on Syria.
Lavrov has echoed Mr. Ban's call for immediate action, saying the sooner an international peace conference on Syria is held, the better. He also called on Syria to allow an inspection for chemical weapons.
Russia has been pushing for inclusion of Syrian ally Iran and western ally Saudi Arabia in those talks, while accusing the United States of trying to narrow the field of participants.
Also Friday, the New York Times reported Friday that Russia has sent more advanced anti-ship cruise missiles to Syria. At the same time, Russia says it has no interest in seeing Mr. Assad remain in power.
On Thursday, U.S.-based group Human Rights Watch said it has evidence that government security branches in the Syrian city of Raqqa hold documents and potential physical evidence that detainees were arbitrarily detained and tortured there. It said its workers gathered the evidence while Raqqa was under the control of rebel fighters earlier this year.
On Thursday, U.S. President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan said they will keep pressure on Syria's president to step down.
Mr. Obama said at a joint White House news conference that he and Mr. Erdogan agree that President Bashar al-Assad should hand over power to a transitional body.
Mr. Erdogan said Turkey and the United States also plan to keep supporting the Syrian opposition and prevent Syria from becoming a terrorist haven.