A U.N. envoy warned Monday of a "horrifying" humanitarian situation brewing in Syria as non-governmental organizations pledged more than $500 million for refugees on the eve of a major donor conference.

The United Nations has launched an appeal to raise $8.4 billion for Syria this year and hopes to receive major pledges at the donor meeting on Tuesday in Kuwait.

The army detained on Monday a Lebanese man on charges of having terror links, the state-run National News Agency reported.

According to NNA, an army patrol raided at dawn the house of Abdullah al-Jaghbari in the area of Bab al-Ramel in the northern city of Tripoli.

The Islamic State of Iraq and the Levant (ISIL) reportedly warned a family residing in the northeastern border town of Arsal to leave the town as soon as possible or they will be executed.

According to As Safir newspaper published on Monday, the family of Ali Ezzedine, who was killed last week by ISIL militants, were compelled to depart their village after jihadists gave them several warnings since Friday, threatening to kill Ezzedine's parents, sister, wife and three kids.

Prime Minister Tammam Salam has stressed that his announcement of Lebanon's backing to any Arab decision which preserves Yemen's stability was the result of his keenness on the unity of the Arab nation.

Addressing the Arab League summit held in the Egyptian Red Sea resort of Sharm el-Sheikh on Saturday night, Salam said Lebanon supported any decision to preserve the territorial integrity of Yemen.

A Lebanese resident of the northern city of Tripoli has died fighting alongside jihadists in the northwestern city of Idlib, media reports said on Sunday.

“Lebanese national Mustafa Khodr was killed in a suicide operation during battles he fought alongside the militants in Idlib,” Lebanon's NBN television said.

Syrian troops regrouped on Sunday after a coalition including al-Nusra Front seized the city of Idlib, the second provincial capital to fall from government control.

The capture is a blow to the government and raises the prospect that the city will become the effective capital of territory held by al-Nusra Front, analysts said.

The United States is expected to grant Lebanon USD 500 million to Lebanon through its United States Agency for International Development (USAID), reported the daily An Nahar on Sunday.

It is supposed to make the announcement at the Kuwait donors conference scheduled for Monday.

Al-Qaida's Syrian affiliate and its allies seized on Saturday the city of Idlib, the second provincial capital lost by the government, the Syrian Observatory for Human Rights said.

After five days of fighting, "Al-Nusra Front and its allies have captured all of Idlib," the Britain-based Observatory said.

Health Minister Wael Abou Faour accused on Saturday the Syrian regime of committing assassinations in Lebanon, expressing fear that the presidential crisis could lead to a “constitutional vortex.”

“I believe that when (Syrian President Bashar) Assad feels that he is militarily comfortable he will have more time to tamper with the situation in Lebanon,” Abou Faour said in an interview with the pan-Arab daily Asharq al-Awsat.

The army's elite forces advanced towards the outskirts of the northeastern border town of Arsal and controlled strategic posts, An Nahar newspaper reported on Saturday.

The daily said that the army controlled during the “unique” military operation, which kicked off at dawn on Friday, advanced posts on the outskirts of the Bekaa border towns Qaa and Nahle after gunmen fled their positions.