U.S. officials say the Obama administration could decide this week on whether to arm Syrian opposition rebels, because of concerns that pro-government forces backed by Iran and Lebanon's Hezbollah militia may be gaining the upper hand in the conflict.

With Syrian President Bashar al-Assad's forces apparently poised for an attack on the strategic city of Homs, a series of White House meetings on Syria has been scheduled for early this week.

Secretary of State John Kerry postponed a planned trip Monday to Israel and three other Middle East countries to participate in the deliberations.

Syrian opposition leaders have warned Washington their rebellion could face devastating losses without greater support.

As many as 5,000 Hezbollah fighters are now in Syria, helping the government push on with its military campaign after capturing the town of Qusair near the Lebanese border last week.

Although a decision has not been made, U.S. officials said President Barack Obama is learning toward sending weapons to vetted, moderate rebel units. Obama already has ruled out any intervention that would require U.S. military forces inside Syria.

Other options, such as deploying American air power to enforce a no-fly zone over Syria, while far less likely, are now being more seriously debated, U.S. media reported.

Any intervention would essentially pit the U.S. and regional allies Saudi Arabia, Turkey and Qatar in a proxy war against Iran, the main arms provider for the Syrian government, and Hezbollah, which is increasingly supplying the manpower.