Discuss as:

A first? Helicopter gunships bombard Syrian capital

AFP - Getty Images

An image grab taken from a video uploaded on YouTube on Sunday shows smoke billowing a neighborhood inDamascus.

Helicopter gunships bombarded several districts of Syria's capital in an effort to drive out insurgents, as rebels battled President Bashar Assad's forces near the main intelligence base in the northern city of Aleppo on Sunday, witnesses said.

"Helicopters are fiercely shelling the northern quarter of Barzeh with rockets and machine guns," an NBC News contact in Damascus said. "They're flying over my head. The shelling is the first of [its] kind ever witnessed in Damascus."

Residents and opposition activists told Reuters that Syria's elite Fourth Division troops were besieging the northern neighborhood of Barzeh, one of three northern areas hit by helicopter fire.  Rebels were also driven from Mezze, the diplomatic district of Damascus, they said. 

The eye-witness report came even as Syrian state television quoted a media source denying that helicopters had fired on the capital. "The situation in Damascus is normal, but the security forces are pursuing the remnants of the terrorists in some streets," it said.

The fourth division is run by Assad's younger brother, Maher Assad, 41, who is widely seen as the muscle maintaining the Assad family's four decades of Alawite minority rule.

Government troops launched an offensive against opposition forces in Syria days after rebels killed some of President Assad's top deputies. NBC's Lester Holt reports.

UN extends Syria observer mission as fighting continues

His role has become more crucial since Assad's defense and intelligence ministers, a top general and his powerful brother-in-law were killed by the bomb on Wednesday, part of a "Damascus volcano" by rebels seeking to turn the tables in a revolt inspired by Arab Spring uprisings in Tunisia, Libya and Egypt.

Assad has not spoken in public since the bombing. Diplomats and opposition sources said government forces were focusing on strategic centers, with one Western diplomat comparing Assad to a doctor "abandoning the patient's limbs to save the organs."

Fighting also raged in parts of Aleppo -- the country's main commercial and industrial hub -- and in Deir al-Zor on the Euprhates river, the largest city in the east.  

London School of Economics Professor Fawaz Gerges discusses what we know about the bomber who killed seven, including five Israeli tourists and himself, then analyzes whether the fall of Syria's President Assad is close.

Death toll hits 19,000
The news of the worsening violence came as an activist group said the death toll had hit more than 19,000 since anti-Assad protests slid into violent clashes in March 2011.

The Syrian Observatory for Human Rights says if the current pace of killing continues through the end of July, it will be the deadliest month since the Syrian uprising erupted.

NBC's Ayman Mohyeldin answers your questions about Syria

Observatory chief Rami Abdul-Rahman said Sunday that 2,752 people — 1,933 civilians, 738 government troops and 81 rebels — were killed in the first 21 days of July.

The average daily death toll in June was 94, while this month it has increased to an average of 131 a day. 

Chemical weapons
As the violence raged seemingly unabated, the United States said it was closely monitoring Syria's chemical weapons stockpile and is "actively consulting" Damascus's neighbors to stress concerns over the security of those weapons and Syria's responsibility to safeguard them.

"We believe Syria's chemical weapons stockpile remains under Syrian government control," White House spokesman Tommy Vietor said on Saturday. "Given the escalation of violence in Syria and the regime's increasing attacks on their people, we remain very concerned about these weapons."

Analysts: Russia will be big loser if Assad falls

That was the White House response to a question about a Syrian military defector's claim that Assad's forces were moving chemical weapons across the country for possible use against the opposition in a military retaliation for the killing of four top security officials.

"In addition to monitoring their stockpiles, we are actively consulting with Syria's neighbors -- and our friends in the international community -- to underscore our common concern about the security of these weapons, and the Syrian government's obligation to secure them," Vietor said. 

General Mustafa Sheikh, who defected recently, has said Assad could now rely only on an inner core of loyal army regiments, saying "the collapse of the regime is accelerating like a snowball." 

Diplomats across the globe voiced their frustrations at the United Nations this morning over the decision by Russia and China to veto a resolution that would have imposed new sanctions on Syria. Amb. Susan Rice discusses.

But Sheikh also said Assad's forces were transporting chemical weapons across the country for possible use against rebel forces.

"The regime has started moving its chemical stockpile and redistributing it to prepare for its use," said Sheikh, citing rebel intelligence obtained in recent days.

His comments could not be verified, but Israel said on Friday it would consider military action if needed to ensure Syrian missiles or chemical weapons did not reach Assad's allies in Lebanon, the Shiite Islamist movement Hezbollah.

"I have instructed the military to increase its intelligence preparations and prepare what is needed so that ... (if necessary) ... we will be able to consider carrying out an operation," Defense Minister Ehud Barak said. 

Reuters and The Associated Press contributed to this report.

More world stories from NBC News:

Follow World News on NBCNews.com on Twitter and Facebook