Where is Syria's president?

• NEW: Russia and China veto a new U.N. Security Council resolution
• NEW: Britain says the two countries are "failing the people of Syria"
• At least 77 people were killed Thursday
• Syria says it repelled terrorist groups trying to enter from Lebanon

(CNN) -- Russia and China vetoed a U.N. Security Council resolution Thursday that would have imposed new sanctions on Syria.

The move came despite heavy diplomatic efforts.

The two countries are "failing the people of Syria," said Britain's U.N. ambassador, Mark Lyall Grant, who was first to speak at the Security Council meeting after the vote. "The effect of their actions is to protect a brutal regime."

Russia and China, which have major trade deals with Syria, have said they want more balanced resolutions that call on all sides to halt the violence.

What's next for Syria's president?

Syria at a tipping point?

Gunfire interrupts Syrian state TV report

A Syrian ambassador defects

Grant accused them of putting "their national interests ahead of the lives of millions of Syrians." And he said they are relying on Syrian President Bashar al-Assad's "broken promises."

Speculation surrounded al-Assad on Thursday, as he remained out of sight a day after a deadly blow to his regime.

Although al-Assad has not often appeared on television or at 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 the president was. The new minister, Gen. Fahd Jassem al-Freij, took his oath in front of al-Assad, state TV said. But it did not show images of the ceremony.

Some reports suggest that al-Assad 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.

Syria, which blames the violence on "armed terrorist groups," said it "repelled" some groups that attempted to enter Syria from Lebanon on Wednesday night.

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 that 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.

As violence has raged in what the Red Cross now declares to be a civil war across the country, world leaders have stepped up diplomatic efforts.

Kofi Annan, joint envoy to Syria for the United Nations and the Arab League, traveled to Moscow this week in hopes of winning support from Russia for tough action at the Security Council.

U.S. President Barack Obama called his Russian counterpart, Vladimir Putin, to discuss the Syrian situation, the White House said in a statement Wednesday.

But Thursday's Security Council vote showed those diplomatic efforts had failed.

Western countries were pushing for a resolution that threatens new sanctions if government forces don't stop attacks against civilians.

The resolution also called for renewing the 300-member U.N. observer mission for 45 days after it was suspended because of violence.

Russia put forward its own draft, which "strongly urges all parties in Syria to cease immediately all armed violence in all its forms."

See the Syria crisis in photos

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.

CNN cannot independently confirm reports of violence in the nation because the government restricts access to foreign journalists.

Attack threatens Syrian president's grip on power

CNN's Arwa Damon, Richard Roth, Brian Walker and Salma Abdelaziz contributed to this report.