The United States plans to boost assistance and weapon shipments to insurgent factions mostly based in Jordan, along Syria's southern border, the officials familiar with the plan told Reuters on Friday.

The additional supplies will not include surface-to-air missiles, the officials said, raising questions over the impact in a foreign-hatched conflict that has killed thousands of people, produced nine million refugees and threatens to destabilize the region.

Rebels have urged the administration of US President Barack Obama to provide advanced weapons including surface-to-air missiles and exert greater military pressure on Syrian President Bashar al-Assad, who has gained several victories in insurgent neighborhoods in recent month.

The United States, however, fears supplies of advanced weapons to pro-Western militants may be diverted to extremist groups, who could use them to attack allied, Israeli or civilian aircraft, the US officials said, explaining why the surface-to-air missiles won't be included in the assistance.

Details of how much aid will flow to the screened insurgent groups are the subject of continuing discussions. It's also unclear, for instance, how much of the military aid would be covert or whether there would be a US military or special forces role.

US allies, including Saudi Arabia and Qatar, have provided weaponry to various militant factions during the conflict, including some extremist groups that are now at odds with the insurgents grouped under the umbrella of the so-called Free Syrian Army.

Syria has been gripped by deadly violence since 2011. Over 150,000 people have reportedly been killed and millions displaced due to the violence fueled by Western-backed militants.

NTJ/MB