US President Joe Biden called Sunday Syria's political upheaval following the ousting of President Bashar al-Assad a "historic opportunity" for Syrians to rebuild their nation.

During a White House statement on the situation in Syria, he said that Washington will "remain vigilant" against the development of terrorist organizations.

Describing it as a "historic opportunity for the long-suffering people of Syria," Biden called the fall of the Syrian regime a "fundamental act of justice."

When asked what should happen to the overthrown president, who is reportedly in Moscow, Biden voiced that he should be "held accountable," expressing that the US will support Syrians in rebuilding their nation, without delving into the details of this support.

"We will engage with all Syrian groups, including within the process led by the United Nations, to establish a transition away from the Assad regime toward independent, sovereign" democratic Syria "with a new constitution," the US president said.

Biden also stated, "First, we'll support Syria's neighbors ... should any threat arise from Syria during this period of transition," further disclosing that senior officials from his administration will visit the region soon.

The US will also ensure stability, particularly in eastern Syria, and protect its personnel "against any threat," while continuing efforts to combat ISIS, including maintaining "the security of detention facilities where ISIS fighters are being held as prisoners," the outgoing US president said.

Acknowledging concerns over rebel groups’ "own grim record of terrorism and human rights abuses," Biden assured vigilance in assessing their actions moving forward.

He also warned that ISIS might try to exploit any power vacuum in Syria but reassured that the US would not permit this, highlighting that US forces recently hit a "dozen strikes" against ISIS in Syria. 

The US maintains roughly 900 occupation troops in Syria under the guise of fighting ISIS.

Biden also acknowledged that US investigators think the American journalist Austin Tice, who went missing in Syria in 2012, is still alive.

"We believe he's alive," Biden added, but the US has yet "to identify where he is."

It is also worth noting that a senior US official told Reuters that Washington is likely to keep its forces in Syria, which includes nearly 900 troops and other assets. 

Ex-Syrian President Assad, family in Russia, granted asylum: Kremlin

Former Syrian President Bashar al-Assad and his family have arrived in Moscow, where they have been granted asylum on humanitarian grounds, a Kremlin source confirmed to RIA Novosti on Sunday.

Russia's Vesti Nedelya newspaper, citing unnamed sources, reported that al-Assad, who stepped down after armed groups, now serving as the transitional government forces of Syria, seized control of Damascus on Sunday, had arrived in Moscow with his family.

In a related context, the Kremlin source said Russia has consistently advocated for a political resolution to the Syrian crisis and seeks UN oversight of renewed negotiations.

"Russia has always supported the search for a political solution to resolve the Syrian crisis. We proceed from the need to resume negotiations under the auspices of the UN," the source stated. 

The source also revealed that Russian officials are in communication with representatives of the armed Syrian opposition, whose leaders have pledged to ensure the security of Russian military bases and diplomatic missions in Syria.

Elsewhere, the Kremlin sources affirmed that Moscow is committed to continuing political dialogue to safeguard the interests of the Syrian people and further strengthen Russia-Syria relations.

Earlier on Sunday, Russian Foreign Minister Sergey Lavrov confirmed on Sunday that "Moscow did not participate in talks on the transfer of power" in Syria.

Lavrov expressed Russia's concern about developments in Syria, stressing that "Russian military bases in Syria are in a mode of increased combat readiness." 

He pointed out that "[Former President Bashar] al-Assad left the presidential post and Syria after negotiations with Syrian conflict participants, giving instructions to transfer power peacefully," calling on all parties to refrain from violence and resolve problems politically.