US Secretary of State John Kerry underlined Wednesday the need for a "genuine cessation of hostilities" in Syria to allow humanitarian aid deliveries and pave the way for transition talks.
Speaking after talks with Norwegian Prime Minister Erna Solberg in Oslo, Kerry said Russia and others involved in the Syria process have to "make a decision" on whether they are "serious" about implementing the ceasefire approved by the UN Security Council.
Russia is the key diplomatic and military ally of the regime of Syrian President Bashar al-Assad.
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.
"It doesn't call for a selective ceasefire, it calls for a nationwide ceasefire and it applies to the Assad regime, just as it applies to the opposition," Kerry told reporters alongside Solberg.
"We believe we can achieve enforcement on both sides, we believe that is the only way to get to the table in Geneva to begin to negotiate a legitimate transition," he added.
"There is a way forward, but it will require good effort in the next week, and the next days," Kerry said.
Kerry said he had raised the cessation with his Iranian counterpart Mohammad Javad Zarif - whose country also wields influence on Damascus - on the sidelines of a closed-door conference for peace negotiators and diplomats hosted by the northern European country.
At the Oslo Forum, Kerry earlier 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.