Syrians fleeing the civil war in their homeland are about to displace Afghans as the world's largest refugee population, the United Nations said Tuesday.

Briefing the General Assembly by video link, U.N. High Commissioner for Refugees Antonio Guterres painted a devastating picture of the humanitarian crisis as a result of the three-year conflict.

The head of the U.N. agency for Palestinian refugees spoke Tuesday of the "shocking" conditions he had seen inside a Syrian camp which has been under siege and bombardment for months.

UNRWA chief Filippo Grandi called for sustained access for aid deliveries to some 18,000 Palestinians who have been trapped under fire in the Yarmuk camp, in south Damascus, with dwindling food supplies.

Nearly 2,000 Syrian refugee children in Lebanon are in danger of starving to death if they do not receive immediate treatment, UNICEF said Tuesday, warning of a looming malnutrition crisis.

"Malnutrition is a new, silent threat among refugees in Lebanon," said Annamaria Laurini, the U.N. children agency's representative in Beirut.

Syria's official al-Qaida affiliate on Tuesday gave rival jihadists a five-day ultimatum to submit to a joint Islamic court after a top operative was killed in a suicide bombing.

The ultimatum issued by Al-Nusra Front to the Islamic State of Iraq and the Levant (ISIL) comes after the killing of Abu Khaled al-Suri, the commander of an Islamist brigade who was close to al-Qaida chief Ayman al-Zawahiri.

Former Guantanamo Bay detainee Moazzam Begg is among four people arrested in Britain on Tuesday on suspicion of Syria-related terror offenses, police said.

Begg, 45, was arrested on suspicion of "attending a terrorist training camp and facilitating terrorism overseas", said West Midlands Police.

Around 400 Tunisian jihadists have returned from fighting in the Syrian war, the interior minister said, sparking fears that the battle-hardened militants could fuel Islamist violence back home.

Since the 2011 revolution, Tunisia has been rocked by sporadic violence linked to Muslim extremists who were suppressed under the regime of ousted strongman Zine El Abidine Ben Ali.

Prime Minister Benjamin Netanyahu said Tuesday Israel was doing all that was "necessary" to ensure its security, following reports of an air raid against Hizbullah targets on the Syrian-Lebanese border.

"We are doing everything that is necessary in order to defend the security of Israel," Netanyahu said at a joint news conference in Jerusalem with visiting German Chancellor Angela Merkel.

Three men and a woman were arrested in Britain on Tuesday "on suspicion of Syria-related terrorism offences", police said.

The four were arrested at their homes in the southwest of Birmingham, Britain's second city, in central England, West Midlands Police said.

An Iranian Parliamentary delegation is expected to arrive in Lebanon on Tuesday on a four-day official visit to discuss with senior Lebanese officials means to help security agencies in their confrontation with uncontrolled terrorism.

According to al-Joumhouria newspaper, the Chairman of the Iranian Parliament's National Security and Foreign Policy Committee Alaeddin Boroujerdi will head the delegation.

President Michel Suleiman has emphasized to his visitors on Monday the need to include the Baabda Declaration in the new government's policy statement, reported the daily al-Joumhouria on Tuesday.

Widely informed sources told the daily that Suleiman will not, “under any circumstances,” accept the elimination of the Baabda Declaration from the statement.