President Barack Obama sounded a cautious note Wednesday on whether the impending cease-fire in Syria would lead to peace talks to end the country's five-year civil war.
"We are very cautious about raising expectations on this; the situation on the ground is difficult," Obama said after meeting at the White House with Jordan's King Abdullah II.
But the American leader added, "If over the next several weeks we can see some lessening of the violence that’s been wracking that country, then that provides us a basis to build a longer-term cease-fire."
He said the warring parties fighting for control of Syria might eventually be able to "move forward on the political transition that ultimately is going to be necessary to bring an end to the civil war in Syria.”
Obama reached agreement earlier this week with Russian President Vladimir Putin on a truce set for Saturday, although it allows for continued attacks on Islamic State targets and forces fighting for Syria's al-Qaida affiliate, the al-Nusra Front.
Syrian President Bashar al-Assad said Wednesday that he was ready to support the cease-fire.
Syria's state-run news agency, SANA, said Assad told Putin in a telephone call that he endorsed the truce. But Syria's main opposition group, the Saudi-backed High Negotiations Committee (HNC), has yet to commit to the U.S.-Russian plan.
Combatants are required to agree to the "cessation of hostilities" by noon Friday local time, 12 hours before the truce is set to take effect.
The HNC said its participation was contingent on the delivery of humanitarian aid and the end of sieges and airstrikes against civilians.
U.S. Secretary of State John Kerry told a congressional hearing in Washington on Tuesday that the truce was the best path yet to ending Syria's civil war, but that "we're not here to absolutely vouch it's going to work."
The top American diplomat said discussions are now under way about what to do if the peace talks fail.
In Moscow, the Kremlin said Putin also talked with Iranian President Hassan Rouhani and Saudi Arabia's King Salman about the impending truce. Its statement said Putin and Rouhani, who both support the Assad regime, "stressed the importance of a further cooperation" between the two countries in fighting terrorist groups in Syria.
Moscow said Salman, who is supporting rebel groups fighting Assad, "welcomed" the truce and voiced a "willingness to work with Russia" to implement it.
Meanwhile, Turkish President Recep Tayyip Erdogan said that Syrian Kurdish militia forces it considers terrorists should also be kept out of the scope of the cease-fire, just like Islamic State and the al-Nusra Front.
Erdogan called the truce to halt five years of fighting that has left 470,000 people dead "positive in principle." But he said the notion that the Syrian Kurdish YPG militia should be given support in Syria because it is fighting Islamic State is a "great lie."
The YPG is a branch of the outlawed Kurdistan Workers' Party (PKK), which has waged a three-decade war against Ankara for control of Kurdish southeastern Turkey. The PKK is recognized as a terror group by the United States and the European Union.
WATCH: U.S. lawmakers question Russian commitment to Syria cease-fire plan