With technical support from Russia, Syria has bolstered its air defenses, posing a threat to U.S. aircraft if America decides to intervene in the war, a U.S. official said Monday.

The official confirmed a report that first appeared in the Wall Street Journal.

U.N. leader Ban Ki-moon made a new plea to Syria on Monday to stop blocking an international inquiry into the alleged use of chemical weapons in the country's conflict.

Ban met the head of the investigation team, Ake Sellstrom, as international suspicions about the use of the weapons grow and on the day designated to remember the victims of chemical weapons attacks.

Unidentified assailants fired two land-to-air missiles at a Russian passenger plane carrying over 150 people when it flew over Syria on Monday, the Interfax news agency reported, citing an informed source in Moscow.

"The Syrian side informed us that on Monday morning unidentified people had fired two land-to-air missiles which exploded in the immediate proximity of a civilian plane belonging to a Russian airline," the source was quoted as saying.

Troops loyal to President Bashar Assad clashed with rebels near Damascus international airport on Monday, the Syrian Observatory for Human Rights watchdog reported.

"Violent clashes have been taking place since this morning between regime troops and fighter brigades (of rebels) near the Damascus international airport and in nearby villages," the group said.

More than 45,000 Syrian refugees have returned to their war-torn country from a northern Jordanian refugee camp over the past nine months, a Jordanian official said on Monday.

Colonel Zaher Abu Shehab, who runs the Zaatari refugee camp, said a total of 45,865 Syrian refugees "have agreed to voluntarily leave the camp and return home" since July 2012, Petra news agency reported.

A land mine explosion seriously wounded two members of Tunisia's national guard on Monday as they pursued a group of armed Islamists near the Algerian border, the interior minister said.

The men were "seriously wounded by the explosion of a mine while they were conducting a search operation at Mount Chaambi targeting terrorist elements on the run," the ministry said, referring to Islamist militants.

The health problems of Algerian President Abdelaziz Bouteflika, who suffered a mini-stroke at the weekend, have intensified speculation about who might succeed him one year ahead of presidential elections.

His hospitalization in France, coming amid press reports of corruption scandals implicating members of his family, have undermined speculation about the 76-year-old president running for a fourth term.

Palestinian Christians near Bethlehem on Monday urged Pope Francis to speak up against an Israeli decision to build its controversial separation barrier on a route they say would cut off their community.

"We cry to your Holiness with a feeling of despair and urgency in order to keep alive our hope that justice and peace is still possible," said an open letter from the Christians of Beit Jala, a town near the West Bank city of Bethlehem.

Influential Muslim cleric Yusuf al-Qaradawi said he will lead a delegation of clerics next week on a support visit to Hamas-ruled Gaza, a local daily reported Monday.

Qaradawi, an Egyptian who is also a citizen of Qatar and close to the Muslim Brotherhood, will travel to Gaza on May 8 along with a delegation of the Doha-based International Union of Muslim Scholars, he told Al-Arab daily.

Parliament speaker Osama al-Nujaifi called on Monday for the cabinet to resign and for early elections to be held, as a seven-day wave of violence killed more than 230 people in Iraq.

The initiative is aimed at "national reconciliation and maintaining the gains of democracy," as well as "sparing the country from the specter of civil war and sectarian strife," Nujaifi's office said in a statement.