Syria's Assad is now in Moscow, Russian state media says
The Syrian government collapsed after a surprise rebel offensive.
Rebel forces in Syria captured the capital Damascus and toppled the regime of President Bashar al-Assad in a lightning-quick advance across the country.
As Assad resigned and fled Syria, rebel forces moved into Damascus as citizens celebrated there and in the streets of Aleppo, where the rebel offensive began 11 days ago.
Meanwhile, the ceasefire in Lebanon is holding despite ongoing Israeli airstrikes on Hezbollah targets, which Israeli officials say are responses to ceasefire violations by the Iranian-backed militant group.
The Israel Defense Forces continued its intense airstrike and ground campaigns in Gaza, particularly in the north of the devastated Palestinian territory.
Tensions also remain high between Israel and Iran after tit-for-tat long-range strikes in recent months and threats of further military action from both sides.
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Celebrations erupt in Syria in the wake of Assad regime's fall
Syrians poured into the streets of Aleppo on Sunday, celebrating the fall of President Bashar al-Assad in a surprise rebel offensive that started 11 days ago.
"An indescribable feeling, without injustice and forbidden things. Things that were forbidden to us only," Aleppo resident Alaa Mayo told ABC News. "We had war, beatings, imprisonment... And our rights, we won't obtain them like the rest of the world."
In Damascus, celebratory gunfire accompanied celebrations since the rebels' overnight takeover of the Syrian capital city. Video showed citizens celebrating in Damascus' Umayyad Square.
ABC News' Camilla Alcini
US mission in Syria 'unchanged,' Pentagon says
Pentagon Press Secretary Maj. Gen. Patrick Ryder said Sunday that the U.S. is monitoring the evolving situation in Syria, where rebel forces have seized Damascus and President Bashar al-Assad's government has collapsed.
"The mission of U.S. forces remains unchanged and focused on the 'Defeat-ISIS' mission," Ryder said. "As always, we maintain the inherent right of self-defense and will take necessary steps to protect our service members deployed to the region."
There are around 900 U.S. troops deployed in eastern and northeastern Syria.
-ABC News' Cindy Smith
Israel strikes 'key weaponry' in Syria as rebels seize control
Israel Defense Forces officials told ABC News that Israel is trying to prevent "strategic abilities" of the fallen government from falling into the hands of rebels now consolidating control after toppling President Bashar al-Assad.
The officials said the IDF struck several "significant" military capabilities and "key weaponry" in Syria "posing a threat to Israel" in recent days.
The IDF officials would not comment on the exact nature of the assets being attacked. Two other regional sources said the targets were related to Syria's residual chemical weapons program, ballistic missiles, aircraft and other advanced assets.
The IDF officials also confirmed that Israeli troops of the 98th Division -- which includes a commando brigade, a paratrooper brigade and specialized infantry units -- were deployed in several locations inside Syrian territory close to the Israeli border.
The units are there to prevent extremists from entering a buffer zone between the two countries originally established in a bilateral 1974 agreement, the officials said. The deployment is intended to be temporary, but the IDF remains in readiness "for all scenarios," the officials said.
-ABC News' Dana Savir and Joe Simonetti
Assad resigned presidency and left Syria, Russia says
The Russian Foreign Ministry said in a Sunday morning statement that Syrian President Bashar al-Assad "decided to leave the presidential post and left the country, giving instructions to transfer power peacefully."
Assad's whereabouts remain unknown as rebels celebrate the fall of his government in Damascus and across the country. The lightning rebel offensive that began last month proved the final chapter in Assad's more than 24 years in power -- and the end of his family's 50-year rule.
"We are following the dramatic events in Syria with extreme concern," the Russian Foreign Ministry said. Assad stepped down "as a result of negotiations" with "a number of participants in the armed conflict," the statement said.
"Russia did not participate in these negotiations," it added. "At the same time, we appeal to all parties involved with a strong call to renounce the use of violence and resolve all governance issues by political means."
Moscow "is in contact with all groups of the Syrian opposition," the statement said, and supports "efforts to establish an inclusive political process based on the unanimously adopted UN Security Council Resolution 2254."
"All necessary measures are being taken to ensure the safety of our citizens in Syria," the ministry said. "Russian military bases in Syria are on high alert. There is currently no serious threat to their security."