Lebanese forces have fired on Syrian aircraft that violated the country's airspace, the first time they have done so since Syria's uprising broke out three years ago, said security officials.
The move suggests Beirut is trying to enforce greater respect for its borders in the hopes of slowing the expansion of the conflict into Lebanon, where it has exacerbated sectarian tensions and prompted shadowy groups to conduct attacks that have killed dozens this year.
Also on Monday, a UN official said at least 15 people have died of hunger-related illnesses in a besieged area of Damascus over the past four months.
Lebanese officials said the military fired anti-aircraft guns at two Syrian helicopters after they fired four projectiles in a mountainous, barren area close to the eastern Lebanese town of Arsal.
Syrian aircraft have frequently conducted strikes near the frontier and sometimes fire has hit Lebanon. Beirut has protested but not shot at them.
A Lebanese military official could not confirm the report, but said the army has orders to shoot anything - planes, tanks or troops - that violated Lebanese territory.
There was no comment from the Syrian government.
Other officials said the Syrians were chasing rebels who were trying to sneak into the country. Lebanon's border towns dominated by Sunni Muslims have become safe shelters for rebels battling the rule of Syrian president Bashar Assad.
Syria's three-year conflict has become increasingly sectarian as it wears on. The rebels are largely from the Sunni majority. Religious minorities, including Shiites, support Assad or have remained neutral, fearing for their fate if Muslim hardliners come to power.
Those loyalties are reflected in Lebanon where Sunnis generally support the rebels, and Shiites support Assad. The Lebanese Shiite group Hezbollah has sent its fighters to Syria to shore up Assad's forces, adding to the tensions.
Meanwhile in Damascus, the new deaths from hunger-related illness highlighted the Assad government's tactic of starving out rebel-held areas.
Rebels seized the Palestinian-dominated Yarmouk district last year, part of a swath of neighbourhoods around Damascus now held by opposition fighters.
The UN's Relief and Works Agency that supports Palestinian refugees had until recently shipped food into the area, but has not been able to do so since September, said the organisation's Chris Gunness. He said at least five people died over the weekend, but 10 people had previously passed away in the months before. The dead include men, women and children.
Mr Gunness estimated 20,000 civilians still lived in the area, where clashes between rebels and Assad loyalists frequently break out.
"If this situation is not addressed urgently, it may be too late to save the lives of thousands of people including children," warned Mr Gunness in a statement to the Associated Press.
Rami Abdurrahman of the British-based Syrian Observatory for Human Rights, which has a network of activists on the ground, also confirmed the numbers. He said those who died over the weekend included an elderly man, a man suffering from unknown disabilities, and a woman.
Even though Yarmouk lies in the heart of the Syrian capital, news of their deaths took weeks to confirm because the area is so tightly sealed, said Mr Abdurrahman.
Yarmouk before the war was a densely populated district of cheaply built multi-storey homes, but was called a "camp" since Palestinians came there as refugees during the 1948 Middle East war surrounding Israel's creation.
Government forces are also besieging other rebel-held areas around Damascus, including the nearby area of Moadamiyeh, where activists said at least two women and four children died of hunger-related illnesses through September.
This week, Moadamiyeh's rebels accepted a humiliating deal where they would receive food in exchange for raising the government flag over the area.
On Saturday, three small pickup trucks entered with bread, rice and canned food; activists said it was not enough.
Also on Sunday, Mr Abdurrahman of the Observatory said the group had received death threats from other anti-Assad activists, including hard-line Islamists, who warned they would kidnap or kill the some 230 people who work for the group inside Syria.
Mr Abdurrahman said the threats escalated after the group reported that hard-liners had attacked minority civilians simply for not being Muslims.
Other activists in northern parts of Syria and Iraq, where the al Qaida-linked Islamic State of Iraq and the Levant (ISIL) is strong, have also complained of threats. Dozens have been kidnapped or forced to flee over the past year, fearing for their lives.
The compliant came as the media rights group Reporters Without Borders warned that ISIL had stepped up attacks on activists.