GEOFF BENNETT: The United Nations today said its peacekeeping forces in Southern Lebanon came under attack today by Israeli forces.

Israel invaded the south last month to press its military campaign against Hezbollah militants.

The U.N. peacekeeping force, which goes by its acronym, UNIFIL, in Lebanon is charged with keeping the border between Lebanon and Israel quiet and demilitarized on the Lebanese side.

As special correspondent Leila Molana-Allen tells us, the prospects for either are further away each and every day.

LEILA MOLANA-ALLEN: Shock in the international community today, as Israeli tanks fired on two U.N. peacekeeper bases in Southern Lebanon.

Two soldiers were injured.

Elsewhere along the southern border, fighting raged between IDF soldiers and Hezbollah forces; 10,000 peacekeepers from 50 U.N. nations are stationed here in the far southern coastal town of Naqoura, with bases dotted along what's called the blue line.

For nearly two decades, UNIFIL has tried to implement the U.N. resolution that aims to keep peace along this febrile border.

In calmer times, UNIFIL troops work to assist the local community here and distribute aid when necessary.

But for the past two weeks, they have been trapped under intense shelling and small-arms fire from both sides.

U.N.

Resolution 1701 was a key part of bringing to an end the last major conflict between Israel and Lebanon in 2006.

It had three key provisions, that Hezbollah would lay down its weapons and withdraw behind the Litani River.

In its place, the Lebanese Armed Forces would become the official authority in the south of Lebanon.

And, in return, Israel would respect the blue line, the border drawn up between the two countries.

In the years since, none of the parties have fulfilled or respected those obligations.

Hezbollah has continued to grow its weapons arsenal.

The Lebanese Armed Forces have never grown the power or weaponry they need to establish authority in the south of Lebanon.

And Israel, even before this latest round of conflict, has launched daily incursions into Lebanese airspace and seawaters.

Tensions have been building since Israel asked the Irish UNIFIL contingent to pull back last week.

They refused.

IDF troops dug in just meters from the UNIFIL base.

Hezbollah accused Israel of using the peacekeepers as human shields.

Today's attacks are a significant escalation.

The U.N. Security Council called an urgent meeting to discuss the crisis.

Ireland's foreign minister, the country with the biggest peacekeeper contingent, condemned the attack.

Andrea Tenenti, UNIFIL spokesperson based at the headquarters fired upon today, called the IDF's attacks a grave violation of international law.

ANDREA TENENTI, Spokesperson, United Nations Interim Force In Lebanon: The shelling has been ongoing between them and Hezbollah, with our base in between.

It's also important to remind the parties that it's their obligation to ensure the security and safety of our troops.

And that's something we have been reminding them.

The shelling continued.

LEILA MOLANA-ALLEN: The IDF says that it is taking care of its responsibility to preserve The safety of U.N. peacekeepers there by telling them to leave for their own security, in the same way it says it's protecting people in Lebanon by telling them to leave their homes.

What's your response to that?

ANDREA TENENTI: It's definitely very, very alarming, and more than alarming, because this is a sovereign country.

Troops from other countries inside Lebanese territory is a violation of the territory.

But we still have to be present.

We are there to try to prevent escalation of the conflict, also assisting local communities with medical activities, but also bringing supply, water, et cetera, which is very much needed at this point, because there are thousands of people stranded, stuck in the south, with very little in terms of water or food.

LEILA MOLANA-ALLEN: Many people internationally will be watching this conflict and wondering, there's a Lebanese army, the Lebanese Armed Forces.

Why are they not on the front lines?

ANDREA TENENTI: The issue with the Lebanese army, that they're extremely committed and also they have very well received by the population in Lebanon.

They also have a lot of difficulties in terms of capacity and capabilities.

It's important for the international community to understand how relevant it is to support the Lebanese army and to bring a large number of troops equipped and trained to the south to bring back state authority.

And we can assist in the implementation, but we need the serious commitment from both sides.

And, at the moment, we do not have it.

But we need it right now.

We need it right now because every day the situation is deteriorating, not only in the south, in Lebanon and in the region.

LEILA MOLANA-ALLEN: U.N. workers and peacekeepers were once a red line for the international community.

But over the past year, as some parties to the conflict have challenged the U.N.'s legitimacy, those lines have blurred.

Multiple U.N. officials voiced frustration to the "News Hour" that respect for international humanitarian law and U.N. authority are on the verge of collapse.

The U.N. and its peacekeepers are in an impossible position.

Their mandate means they can't fight, so if they stay put and these attacks continue, they will be sitting ducks.

If they leave, what could be the international community's last chance to stop this conflict in its tracks goes with them.

Nearly 20 years after the world came together to find a lasting resolution, the same old battle lines have exploded into a war already more bitter and bloody than the last.

For the "PBS News Hour," I'm Leila Molana-Allen in Yarze, Lebanon.