The UN Security Council has expressed "deep concern" at new evidence that illegal arms are being smuggled into Lebanon from neighbouring Syria.
The UN is currently monitoring the situation in Lebanon and Syria
It issued a statement after regional envoy Terje Roed-Larsen said weapons and fighters often cross the border.
Mr Roed-Larsen described the build-up of militias as the "greatest obstacle" to stabilising Lebanon.
The Security Council also backed the Lebanese army's handling of a militant siege in a Palestinian refugee camp.
Under the terms of UN resolutions that ended last summer's 34-day conflict in Lebanon, private militias operating within Lebanon are required to disarm.
That ruling applies to Palestinian militant groups such as Fatah al-Islam, battling the army at the Nahr al-Bared refugee camp, as well as the Shia militant group Hezbollah.
In its statement, the Security Council said it regretted that both Lebanese and non-Lebanese militia - like the Palestinians - had not yet been compelled to disarm.
Mr Roed-Larsen was previously the senior UN envoy for the Middle East region but now has specific responsibility for Lebanon and Syria.
His downbeat assessment of the military balance and situation in the two countries prompted the Security Council to speak out.
In the statement, the Security Council said it was looking forward to the conclusions of a team currently monitoring security in the border area.
"It [the council] reiterates its call for the strict respect for Lebanon's territorial sovereignty and territorial integrity," the statement said.
Speaking after briefing the council, Mr Roed-Larsen said increased evidence that fighters and weapons were crossing into Lebanon was "alarming and deeply disturbing".
He also told reporters that the general picture of the situation in the Middle East was "very dark, and apparently getting darker".
He said the emergence of new and interlinked issues - such as Iraq, Iran and Syria-Lebanon - was complicating efforts to promote peace.
"The new phenomenon seems to be that all these conflicts are now completely intertwined so that it is very difficult, maybe impossible, to find a solution to one of them without finding a solution to all of them."