BEIRUT - The Lebanese army fired on Syrian aircraft that violated the country's airspace Monday, the first time Lebanon has done so since Syria's uprising broke out nearly three years ago, security officials said.

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 Monday, a U.N. official said at least 15 people have died of hunger-related illnesses in a besieged area of Damascus over the last four months.

Lebanese officials said the military fired antiaircraft guns at two Syrian helicopters after they fired four missiles in a mountainous, barren area close to the eastern Lebanese town of Arsal.

Syrian aircraft have frequently conducted strikes near the frontier, sometimes hitting Lebanese territory. Beirut has protested but not responded with force.

The officials spoke on condition of anonymity because they weren't authorized to speak to the media.

A Lebanese military official could not confirm the report, but said the army has orders to shoot anything - planes, tanks or troops - that violate Lebanese territory. The official spoke on condition of anonymity in line with policy.

There was no comment from the Syrian government.

Local security officials said the Syrians were chasing rebels who were trying to sneak into Lebanon. Communities on the Lebanese side of the border dominated by Sunni Muslims have become safe havens for rebels battling the rule of Syrian President Bashar al-Assad.

Syria's three-year-old conflict has grown 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.

In Damascus, meanwhile, the new deaths from hunger-related illness highlighted what activists say is the Assad government's tactic of starving out rebel-held areas.

Rebels seized the Palestinian-dominated district of Yarmouk last year, part of a swath of neighborhoods around Damascus now held by opposition fighters.