Both Moscow and Washington say they are close to reaching a cease-fire deal on Syria but still require more time.
Russian Deputy Foreign Minister Sergei Ryabkov said on September 4 that "We are close to the deal.... But the art of diplomacy requires time to implementation."
He added, "I think we have no reason to expect that talks will collapse."
He was speaking to media on the sidelines of the Group of 20 economic summit in the Chinese city of Hangzhou.
Earlier in the day, U.S. President Barack Obama said in Hangzhou that talks with Russia will be key in reaching any deal to end hostilities in Syria but negotiations are difficult.
"We have grave differences with the Russians in terms of both the parties we support but also the process that is required to bring about peace in Syria," Obama said.
"We're not there yet," he added. "I think it's premature for us to say there's a clear path forward, but there's the possibility at least for us to make some progress."
U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov have been deep in talks for weeks over a deal to boost U.S. and Russian military cooperation to fight the Islamic State group and other extremists in Syria.
Speaking to reporters on September 4 after meeting with Lavrov, Kerry said a couple of tough issues still remained, adding that the two sides would meet again on September 5.
"It's fair to say that out of the review I think there are a couple of tough issues that we talked about today," Kerry said, declining to give details.
"We will meet tomorrow morning and see whether or not it is possible to bridge the gap and come to a conclusion on these couple of issues," Kerry added.
He added that the United States will not rush into a deal with Russia to end the civil war in Syria.
A possible deal could include provisions to ensure aid can reach besieged areas of Syria and steps to prevent Syrian President Bashar al-Assad's government from bombing areas where U.S.-backed rebels are operating.
Washington's Syria envoy, Michael Ratney, has said the deal would also require the withdrawal of Damascus's forces from a key supply route north of Aleppo, according to a letter from Ratney to the Syrian armed opposition seen by Reuters and dated September. 3.
In return, the United States would coordinate with Russia against Al-Qaeda, the letter said, without elaborating.
The civil war has killed more than 250,000 people and displaced 11 million, causing a refugee crisis in the Middle East and Europe, and contributing to a rise in militant Islamist groups.
Russia has backed Assad, but the United States has worked with moderate opposition forces fighting Assad.