Activists said Tuesday that at least 15 opposition fighters have been killed in a battle with Syrian government troops for a military helicopter base in the country's north.

The Britain-based Observatory for Human Rights said fighting between rebels and President Bashar Assad's troops is raging around the Mannagh base in the northern province of Aleppo.

Hizbullah commander Hussein Habib's family is still mourning his death in Syria, where he fought alongside regime soldiers. But they say they are ready to offer another son up as a "martyr".

In their town of Baalbek and other Hizbullah strongholds in eastern Lebanon, it's no longer a secret that the group's members are crossing the border to bolster the ranks of government troops battling rebels.

A former Israeli defense minister has said that Syria's chemical weapons are "trickling" to Hizbullah. But Israeli officials said they did not have any evidence about the alleged transfer.

Binyamin Ben-Eliezer, a retired general who is now a lawmaker from the opposition Labor party, told The Associated Press on Monday: "The process of weapon transferal to Hizbullah has begun."

Speaker Nabih Berri slammed on Tuesday Lebanese officials for seeking to adopt an electoral law that help them achieve the parliamentary majority, pointing out that the cabinet could be formed ahead of a parliamentary session set to be held on May 15.

Berri said in comments published in As Safir newspaper that his hybrid electoral law proposal calls for electing half of the lawmakers in the new parliament based on the winner-takes-all system, with the other half elected under the proportional representation system and 26 districts.

U.S. President Barack Obama stepped up pressure on Russia over Syria on Monday, telling his Russian counterpart Vladimir Putin of his concern about the reported use of chemical weapons by the Damascus regime.

Obama also thanked Putin in the telephone call for his help after the Boston marathon bombings, and expressed condolences over a fire that killed 36 patients in a Russian psychiatric facility on Friday, the White House said.

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.

French Foreign Minister Laurent Fabius reiterated on Monday his country's support for Lebanon's policy of disassociation towards Syria's conflict, vowing to provide aid in hosting the Syrian refugees.

"We are aware of the Lebanese efforts made in rescuing the refugees and we pledge to help in this respect,” Fabius said after meeting with caretaker Prime Minister Najib Miqati in Paris.

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.

British Minister for the Middle East Alistair Burt on Monday called on the Lebanese to refrain from sending fighters to war-torn Syria, reiterating “the importance of international community support to Lebanon's disassociation policy.”

“As I discussed with Speaker (Nabih) Berri, President (Michel) Suleiman and the Army Commander (General Jean Qahwaji) today, I am pleased that we are, together, building Lebanon’s capacity to protect its sovereignty,” Burt said after talks with Berri in Ain al-Tineh.