US Secretary of State John Kerry says that the United States, Russia, and Iran are all "responsible" for ensuring that both the opposition group and the Syrian government live up to the Syria truce.
"We have a responsibility to make certain that the opposition lives up to this, and Russia and Iran have a responsibility to make sure that the Assad regime lives up to this," Kerry told reporters in Paris on Monday.
Iran has time and again stressed the need for a political solution to the crisis, noting the fate of the Syrian government must be determined by Syrians only.
The truce, brokered by Russia and the United States, went into effect late February in a bid to facilitate negotiations between warring sides to the conflict.
On Monday, the US reached a new agreement with Russia, which co-chairs the International Syria Support Group (ISSG) together with Washington.
Based on the agreement, the two sides confirmed their commitment to the existing ceasefire in Syria and promised to intensify their efforts to ensure its implementation.
Kerry said the new agreement could ultimately "reinstate a nationwide cessation of hostilities.”
The secretary of state, however, noted that "the proof will be in the eating of the pudding, " adding, "these are words on a piece of paper. They are not actions."
"It is going to be up to the commanders in the field and the interested parties — which includes us," he noted.
Tehran has already said that its red line is Syrian President Bashar al-Assad’s remaining in power until his term in office is over.
“Since the Syrian nation chose Bashar al-Assad as president two years ago, he will remain in the post until the Syrian people change him,” Ali Akbar Velayati, a senior adviser to Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei, said on Friday.
Velayati said the Syrian nation and government are firmly defending their country, rejecting the idea of imposing a president on Syria who would serve the interests of Saudi Arabia or any other party.
Assad has said his country will accept nothing less than “final victory” over the foreign-backed militants and the terrorists in Syria.
On Thursday, a 48-hour ceasefire took hold in Syria's largest city, Aleppo, with residents returning to the streets as fighting subsided.
Aleppo has been divided between government forces in the west and militants in the east since 2012, a year after the conflict broke out in Syria.
Syria is grappling with foreign-backed militancy, including Daesh and al-Qaeda-affiliated al-Nusra Front. The United Nations special envoy for Syria Staffan de Mistura estimates that over 400,000 people have been killed in the conflict, which has also displaced over half of the Arab country's pre-war population of about 23 million.