- At least 77 people were killed Thursday
- U.N. Security Council deliberations have been postponed until later Thursday
- Syria says it repelled terrorist groups trying to enter from Lebanon
- "Anything that moves is currently being shelled," opposition activist says
(CNN) -- A day after a devastating, deadly blow to his regime, the Syrian president has not been seen or heard from publicly.
Although Bashar al-Assad has not often appeared on television or in public events during the near 17-month crisis gripping his country, it is unusual for a leader not to address a nation in the wake of a major bombing -- and as violence rages in the capital city.
It is particularly striking in al-Assad's case, as world leaders and analysts call Wednesday's attack a sign that he is losing control over his country.
The blast at a government building in Damascus killed three top officials, one of whom is also the president's brother-in-law.
State TV reported that al-Assad quickly replaced the slain defense minister, but it did not say where al-Assad was.
Speculation is surrounding his whereabouts, with some reports suggesting he may have left Damascus.
At least 77 people were killed Thursday, including 25 in Damascus suburbs and 13 inside Damascus, the opposition Local Coordination Committees of Syria said. The Damascus suburb Qaboon was under heavy fire Thursday, according to an opposition activist.
Video: Fighting rages around Damascus
Residents trying to flee the violence have nowhere to go because clashes are raging in most of Damascus province, said Omar al-Dimashki, a spokesman for the Revolution Leadership Council of Damascus.
"It is surrounded by tanks, and anything that moves is currently being shelled and rockets are falling on the homes," he said.
The U.N. Security Council is preparing to vote on a resolution aimed at bringing peace to the country.
The council's scheduled deliberations were postponed until later Thursday "because of the dramatic developments in Syria," the United Nations said. Secretary-General Ban Ki-moon hopes the council members "shoulder their responsibility and take collective and effective action with a sense of urgency," a U.N. statement said.
Syria, which blames the violence on "armed terrorist groups," said it "repelled" some groups that attempted to enter Syria from Lebanon on Wednesday night.
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Gunfire interrupts Syrian state TV report 
A Syrian ambassador defects The regime also said Wednesday's bombing was carried out by people "implementing foreign plots."
The Syrian military issued a statement stressing "resolution to decisively eliminating the criminal and murder gangs and chasing them out of their rotten hideouts wherever they are until clearing the homeland of their evils," state-run news agency SANA said.
Anyone who thinks targeting leaders will "twist Syria's arms is 'deluded,'" the statement said, according to SANA.
Video: Syria government says it will fight back
Officials killed were the defense minister; the deputy defense minister, who is the president's brother-in-law; and al-Assad's security adviser and assistant vice president, state TV reported.
The deputy head of the rebel Free Syrian Army, Col. Malek al-Kurdi, said the attack was coordinated by rebel brigades. But some other rebel commanders say it's unclear who was behind the attack.
World leaders issued widespread calls for an end to the bloodshed.
Kofi Annan, joint envoy to Syria for the United Nations and the Arab League, traveled to Moscow earlier this week in hopes of winning support from Russia for tough action at the security council.
Russia and China, permanent council members with veto power, have trade deals with Syria and have blocked tough U.N. draft resolutions against the government, leading to accusations that Russia is protecting the Syrian regime.
The two countries have said they want a balanced resolution that focuses on getting all sides to stop the violence.
U.S. official: Al-Assad faces 'fight or flight' decision
Western countries are pushing for a resolution that threatens new sanctions if government forces don't stop attacks against civilians.
The resolution also calls for renewing the 300-member U.N. observer mission for 45 days after it was suspended because of violence.
Russia has put forward its own draft, which "strongly urges all parties in Syria to cease immediately all armed violence in all its forms."
U.S. President Barack Obama called his Russian counterpart, Vladimir Putin, to discuss the Syrian situation, the White House said in a statement Wednesday.
"They noted the differences our governments have had on Syria, but agreed to have their teams continue to work toward a solution," it said.
See the Syria crisis in photos
CNN cannot independently confirm reports of violence in the nation because the government restricts access to foreign journalists.
Since the crisis began in March 2011, the United Nations estimates more than 10,000 people have been killed in the violence; the opposition Local Coordination Committees of Syria estimates that more than 16,000 have died.
Attack threatens Syrian president's grip on power
CNN's Arwa Damon, Josh Levs, and Salma Abdelaziz contributed to this report.
