US Secretary of State John Kerry said Wednesday there was need for a "genuine cessation of hostilities" in Syria to allow humanitarian aid deliveries and pave the way for transition talks.
Kerry also urged Iran to use its influence on Damascus during a visit to Norway where he met his Iranian counterpart Mohammad Javad Zarif on the sidelines of a closed-door conference for peace negotiators and diplomats hosted by the northern European country.
"So this is a critical moment and we are working very, very hard to see if we can, literally in the next week or two, come to an agreement that has the capacity to more fully implement a ceasefire across the country," Kerry told the Oslo Forum delegates.
"I am not going to make any promises that can't be delivered on, but I do believe that the conversation I had with Zarif indicates to me possibilities for how this could be achieved," he added.
In February, a major ceasefire, brokered by the US and Russia, went into effect in most of Syria. The truce later crumbled with the Syrian opposition and the regime trading blame for its collapse.
Russia is the key diplomatic and military ally of the regime of Syrian President Bashar al-Assad.
Kerry cautioned that "Russia needs to understand that our patience is not infinite, in fact it is very limited now with respect to whether or not Assad is going to be held accountable."
"Meanwhile we are also prepared to hold accountable members of the opposition who have been both playing off each other to continue the violence and break the cessation," he added.
The US is leading an international air campaign in Syria against extremists, mainly Islamic State.
The alliance is providing an air cover to an ongoing offensive by an Arab-Kurdish force against Islamic State in northern Syria.
The Syrian government on Wednesday claimed that German and French special forces were illegally present on its territory.
The Syrian Foreign Ministry called the alleged military presence a "flagrant and unjustified aggression" against the country's sovereignty, state news agency SANA reported.
"The effective and legal combat of terrorism requires cooperation with the legitimate Syrian government," the Foreign Ministry said in a statement.
The German Defence Ministry has denied the allegations and said the reports are incorrect.
Damascus' allegations follow claims by some media and activists that German and French militaries are based in northern Syrian areas where the US-backed Democratic Forces of Syria (DFS) - an alliance of Syrian Kurds and Arabs - is fighting to retake the strategic town of Minbij near the Turkish border from Islamic State.
Pro-Damascus broadcaster al-Mayadeen, based in Lebanon, claimed on Tuesday that dozens of German special forces had entered northern Syria and joined French and US troops there.
Meanwhile, the Syrian Observatory for Human Rights, a monitoring group, said that German military advisors and French forces were supporting the DFS in the battle for Minbij.
Idriss Nassan, a Kurdish official in northern Syria, told dpa: "I can confirm the presence of French and US troops, but cannot confirm there are Germans."
On May 31, the DSF started an offensive to seize Minbij, which has been under Islamic State control since 2014.