Syria's Assad is now in Moscow, Russian state media says

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

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."

The Iranian embassy in Damascus has been stormed and looted by Syrians celebrating the fall of President Bashar al-Assad's government, which for several years was heavily reliant on Iranian and Russian backing, state-run media in Tehran reported.

Iran's state-owned PressTV said the mission was ransacked by "militants led by Hayat Tahrir al-Sham," one of the rebel groups leading the surprise campaign that toppled the Syrian government.

Videos from the scene showed people tearing down posters outside the embassy that bore images of former Islamic Revolutionary Guard Corps Quds Force commander Qassem Soleimani and former Hezbollah leader Hassan Nasrallah.

The footage also showed people carrying items out of the mission and smashing its windows.

-ABC News' Joe Simonetti

Geir Otto Pedersen, the United Nations' special envoy for Syria, said in a statement Sunday that the fall of President Bashar al-Assad's government "marks a watershed moment" in the country's history after "nearly 14 years of relentless suffering and unspeakable loss."

"This dark chapter has left deep scars, but today we look forward with cautious hope to the opening of a new one -- one of peace, reconciliation, dignity and inclusion for all Syrians," Pedersen said in a statement.

"The challenges ahead remain immense and we hear those who are anxious and apprehensive," Pedersen added. "Yet this is a moment to embrace the possibility of renewal. The resilience of the Syrian people offers a path toward a united and peaceful Syria."

Pedersen called for "stable and inclusive transitional arrangements" to smooth the transfer of power on the path to "a unified Syria, with its sovereignty, independence and territorial integrity, in a way that can receive the support and engagement of the entire international community."