Russia threatened Monday to take military action against violators of the Syrian ceasefire, highlighting the fragility of the current situation.
Russia and the US, which support opposing sides in the Syrian civil war, brokered the ceasefire, which has been the basis for restarting UN-brokered peace talks in Geneva last week.
Russian officials in recent days have announced that there have been about 250 violations of the pact since it came into effect last month, with at least 60 civilians killed.
If Russia gets no response from the US on its proposals for ensuring the ceasefire pact, it will start applying the agreement's rules unilaterally on March 22, General Sergei Rudskoi said Monday in comments carried by state news agency TASS.
"We emphasize that military force will be used upon obtaining credible evidence of an armed group systematically violating the commitments," said Rudskoi, head of the Russian military's Main Operations Directorate.
At the Geneva talks, UN mediator Staffan de Mistura said he was confident that Russia and the US would overcome the ceasefire issue.
He noted that both countries have been running a joint operations centre to monitor the truce.
"The moment they don't talk substantively, we go back to the past. We can't afford it, and they know it, too," he said.
However, the UN diplomat warned that the future of the ceasefire in Syria depends on progress at the peace talks.
De Mistura received a negative response Monday when he urged Syrian government envoy Bashar al-Jaafari to enter into talks about political transition immediately.
"He said it was premature at the moment to talk about it," reported de Mistura, who is conducting the indirect talks by shuttling between the government and opposition delegations.
The truce had been largely holding and it was accompanied by a surge in aid deliveries, de Mistura said, "but neither of the two can be sustained if we don't get the progress on the political transition."
While the opposition has demanded the resignation of President Bashar al-Assad, al-Jaafari insisted Monday the government would not discuss such matters.
"The president has nothing to do with the Syrian-Syrian indirect talks," he told reporters.
On Friday, Russian general Rudskoi said Russia's air force would continue bombing UN-designated terrorist organizations in Syria, specifically Islamic State and the al-Qaeda affiliated al-Nusra Front.
The ceasefire does not extend to these groups.
Days earlier, President Vladimir Putin had ordered the Russian military to withdraw the majority of its forces from Syria, saying their task had been fulfilled.
Russia began a bombing campaign in Syria six months ago to provide support to President Bashar al-Assad, a longtime ally.
Al-Assad's military has regained significant territory with Russia's help. Russia has said that the main purpose of its campaign in Syria has been to stamp out terrorism and restore stability.