The US could make a decision as early as this week on whether to arm Syrian rebels.

An official, who spoke on condition of anonymity, said the question of whether to arm rebels is on the agenda of White House meetings during the early part of this week.

Separately, an Obama administration source said that US Secretary of State John Kerry has put off his planned trip to the Middle East in order to attend the meetings in Washington.

The renewed focus comes two years into the uprising against Mr Assad that has sparked civil war and seen at least 80,000 people killed.

Mr Assad, whose Alawite minority is an offshoot of Shia Islam and whose family has ruled Syria for more than four decades, is backed by Iran and Lebanon's Hezbollah.

There is increasing concern that Mr Assad may be gaining the upper hand in the conflict as forces loyal to the government recently captured the key town of Qusair.

The US and other governments are also weighing evidence that Mr Assad's forces may have used chemical weapons, something US President Barack Obama has said would cross a "red line".

Meanwhile, al-Qaeda leader Ayman al-Zawahri has intervened in a dispute between the Iraqi and Syrian branches of his network.

He has told both sides to "stop arguing".

Al-Qaeda in Iraq announced in April that it had united with Syria's Nusra Front, which spearheads the fight against President Mr Assad.

This upset Nusra, which affirmed its loyalty to Mr Zawahri, but said it had not been told of any merger.

Nusra Front leaders are aware that many Syrians had joined the front because of its military prowess rather than for ideological reasons.

The group had previously sought to minimise the use of tactics, such as indiscriminate attacks on civilians, and Islamist crackdowns, which had alienated many Iraqis from al-Qaeda in Iraq.

The al-Qaeda leader annulled the merger declared by the leader of the Islamic state in Iraq saying each group was separate.

He also said Baghdadi and Nusra Front leader Abu Mohammad al-Golani would continue to head their groups for a year, pending a decision by their respective consultative assemblies.