Turkey is to pick a new long-range air defense missile system soon, as tension builds with neighboring Syria, media and foreign diplomats said Friday.

Companies vying for the four billion dollar (3.2 billion euro) contract include Raytheon and Lockheed Martin of the United States, Rosoboronexport of Russia, CPMIEC of China and the French-Italian consortium, Eurosam, they said.

The head of the supreme military council of the rebel Free Syrian Army said on Friday that Syrian troops were massing near the Turkish border amid tensions since Syria's downing of a Turkish jet last week.

"There are military units massing 15 kilometers or slightly more (10 miles), from the Turkish border, in the northern region" of Syria, Mustafa al-Sheikh told Agence France Presse by telephone.

At least 43 people were killed in Syria on Friday as regime forces pounded the Damascus suburb of Douma and the central city of Homs, after one of the bloodiest days of the Syrian uprising in which more than 180 people were killed, monitors said.

The Local Coordination Committees, the main activist group spurring protests on the ground, said regime forces killed 30 people across the country, among them eight in Deir Ezzor, six in Homs, four in Idlib, four in Aleppo, three in Daraa, two in the countryside around Damascus, one in Jableh, one in Damascus and another in Hama.

U.N.-Arab League envoy Kofi Annan will demand "irreversible" steps by President Bashar al-Assad and the Syrian opposition to end the country's conflict, according to a copy of his transition plan obtained by AFP.

A blueprint for multi-party elections and an offer for "significant" international funding to rebuild Syria will also be discussed at a meeting of foreign ministers from the major powers and other key countries in Geneva on Saturday.

The United Nations doubled its appeal for aid funds on Thursday to help thousands of refugees fleeing the violence in Syria, the majority of whom are women and children.

"Around 75 percent of Syrian refugees are women and children. Most are entirely dependent on life-saving humanitarian assistance," said Panos Moumtzis of the U.N. High Commissioner for Refugees (UNHCR).

U.S. Secretary of State Hillary Clinton arrived in Saint Petersburg on Thursday, for talks with her Russian counterpart Sergei Lavrov that are expected to be dominated by the ongoing crisis in Syria.

The two top diplomats are to meet late Friday, the eve of crunch talks in Geneva that will try to find a diplomatic solution to the conflict that has killed an estimated 15,000 people.

Senior officials from countries taking part in international talks on Syria will meet on Friday in Geneva, a spokesman for peace envoy Kofi Annan said.

"There will be a preparatory meeting of the Action Group on Syria tomorrow," the day before planned ministerial talks, Ahmad Fawzi said in a statement on Thursday.

French Foreign Minister Laurent Fabius will meet the head of the opposition Syrian National Council, Abdel Basset Sayda, on Friday before international talks in Geneva on Syria's conflict, the ministry said.

It will be the first meeting between Fabius and Sayda since he was chosen to lead the umbrella Syrian opposition group on June 10.

The former chief of the main opposition Syrian National Council, Burhan Ghalioun visited Beirut during the past two days “on a secret visit”, revealed al-Jadeed television on Thursday.

It said that he held talks with a number of Lebanese officials away from the media spotlight.

A senior Hamas member has been killed in a suburb of the Syrian capital Damascus, an official of the group told AFP on Thursday, accusing Israel's spy agency of carrying out the attack.

Israeli Defense Minister Ehud Barak responded coyly to the suggestion, telling army radio: "I'm not sure that that's necessarily right."