Israeli warplanes bombed the outskirts of Damascus early Sunday for the second time in recent days, according to Syrian state media and reports from activists, signaling a sharp escalation in tensions between the neighboring countries that had already been exacerbated by the conflict raging in Syria.

Videos posted on the Internet by activists showed a huge fireball erupting on Mount Qassioun, a landmark hill overlooking the capital on which the Syrian government has deployed much of the firepower it is using against rebel-controlled areas surrounding the city.

The official Syrian Arab News Agency said that a scientific research facility had been struck by an Israeli missile, and a banner displayed on state television said the attack was intended to relieve pressure on rebel forces in the embattled eastern suburbs. The banner was accompanied by martial music and footage of Syrian soldiers marching, descending from helicopters and firing rockets, indicating that Syria may not shrug off the assault, as it has with some Israeli strikes in the past.

“The Israeli aggression comes at a time when our armed forces are scoring victories against terrorism and al-Qaeda gangs,” state television said.

A subsequent video suggested further strikes were taking place in the same location, although the number was unclear.

An anonymous intelligence official in the Middle East confirmed state media reports that the strikes were carried out by Israeli warplanes, according to the Associated Press. But Reuters news service reported that an Israeli military spokeswoman said, “We don’t respond to this kind of report.”

The attack Sunday came hours after U.S., Israeli and Lebanese officials said Israeli warplanes had struck on Friday a shipment of missiles destined for Lebanon’s Hezbollah movement at Damascus International Airport.

The attacks coincided with mounting pressure on the Obama administration to formulate a response to the growing risk of weapons proliferation in the Syrian war, notably the possibility that chemical weapons are being used in the conflict and could fall into the hands of extremists.

It also came amid renewed reports of sectarian violence in the northern coastal region of Latakia, a stronghold of Syrian President Bashar al-Assad where his supporters allegedly killed at least 50 and perhaps as many as 100 Sunni Muslim villagers in recent days, drawing a sharp condemnation Saturday from the State Department.

Israeli officials told the Associated Press and Reuters that the target of the Friday airstrike was a consignment of advanced, long-range, ground-to-ground missiles destined for Hezbollah, the political and military organization that dominates Lebanon’s government and has been designated a terrorist organization by the United States.

The shipment did not contain chemical weapons, but the missiles were potentially “game-changing,” one official told the Associated Press.

Details of that attack were sketchy, but it appeared the target was a storage site at an air defense base on the periphery of the Damascus airport, known to be the chief transshipment point for weapons flown into Syria from Iran, both to aid the Syrian government in its battle against rebels and to supply Iran’s ally Hezbollah in Lebanon.

A senior Lebanese security official who was in Damascus at the time said the strike took place about 4 a.m. and targeted a large quantity of missiles stored at the site. The official, who, like the others, spoke on the condition of anonymity because of the sensitivity of the subject, did not know the origin or the destination of the missiles.

There were reports Friday that an overnight rebel mortar attack had caused a huge blaze at the Damascus airport, with a video posted online showing at least two locations on fire. But the Lebanese security official said the blasts, which woke him up, were bigger than those caused by mortar shells and that his Syrian counterparts had confirmed to him that the source was an Israeli strike.

The attack appeared to be similar to one in January in which Israeli jets hit a convoy carrying weapons intended for Hezbollah, the official said, with the warplanes firing from a location over Lebanon’s Bekaa Valley.

His claims could not be independently confirmed, but a Syrian opposition Web site also said that the Damascus airport was the target, according to Israel’s Haaretz newspaper. Lebanese authorities and residents had reported unusually intense Israeli overflights during the previous 48 hours, suggesting the warplanes may have struck their target from Lebanese airspace.

A U.S. official in Washington confirmed that the strike had taken place but refused to provide details. Spokesmen for Israeli Prime Minister Benjamin Netanyahu and the Israel Defense Forces declined to comment on the reports.

Israel has also not officially confirmed that it carried out the January strike, on a convoy reportedly transporting antiaircraft missiles to Hezbollah, and the fact that some officials swiftly acknowledged U.S. reports of this attack pointed to Israel’s growing determination to directly confront the threat posed by the Syrian conflict.

Netanyahu and military and intelligence commanders in Israel have repeatedly warned that they will not tolerate the transfer of sophisticated weaponry to Hezbollah under cover of the turmoil of the Syrian war, and they have also expressed certainty in recent weeks that Assad has used chemical weapons in at least two small-scale attacks. One concern is that Syria and Iran will supply Hezbollah with a chemical-weapons capability.

But there are also broader fears that the Syrian war will trigger a revival of the long-standing hostilities between Israel and Hezbollah, which fought a fierce but inconclusive war in 2006 that killed 1,200 people. Many in Lebanon and Israel have long predicted a replay of Israel’s effort to vanquish the Shiite militia that threatens its northern border, and Hezbollah’s apparent efforts to boost its arsenal suggests it is preparing for such an eventuality.

Israel’s chief worry is that a desperate Syrian regime might seek to ensure its survival by using Hezbollah to lash out with an attack against Israel, in fulfillment of Assad’s repeated warnings that his fall would generate regional chaos.

The main concern for the Shiite Hezbollah movement is that the collapse of the Syrian regime in Damascus and its replacement by one led by the overwhelmingly Sunni opposition will undermine its dominant role in Lebanon and leave it vulnerable to Israeli attack. The movement has long relied on Syria for the transshipment of arms supplied by its chief ally, Iran, and the fall of Assad would compromise its supply routes.

The State Department on Saturday condemned the latest example of sectarian violence, saying in a statement that it was “appalled” by the killings this past week in Baida, outside the town of Baniyas, in Assad’s native Latakia province.

Government forces and Alawite irregulars known as shabiha attacked the area with mortar fire, “then stormed the town and executed entire families,” the statement said.

It added, “We will not lose sight of the men, women and children whose lives are being so brutally cut short.”

On Saturday, hundreds of Sunnis fled the area around Baniyas after reports of another incident overnight Friday, in which at least eight deaths have been confirmed, according to the Britain-based Syrian Observatory for Human Rights. A video posted online showed the bloodied bodies of a man, several children and a baby with blackened legs.

Also Saturday, Assad made his second public appearance in three days, visiting Damascus University to inaugurate a statue dedicated to students who have died in the violence. Footage aired by state television showed him being mobbed by cheering, waving supporters.

Assad rarely appears in public, and his visibility this past week suggests his confidence has been buoyed by recent gains by his forces in some parts of the country and by indications that the international community remains reluctant to involve itself in the Syrian conflict, despite the reports that his regime has used chemical weapons.

William Booth in Jerusalem and Anne Gearan, Greg Miller and Ernesto Londoño in Washington contributed to this report.