Citing unnamed US officials, the newspaper said the efforts are aimed at getting Iraq to close its airspace to flights between Iran and Syria flights that US intelligence believes are carrying arms for Syrian pro-government forces.

The report came as government forces in Syria mounted an offensive in the Damascus neighbourhood of Barzeh, triggering an exodus of residents, as a rebel commander appeared in a video saying the battle to "liberate" Aleppo had begun.

The official SANA news agency announced that government forces had "cleansed" the capital's Qaboon neighbourhood of "terrorists", the regime's term for rebel fighters.

And state television aired footage reportedly from Qaboon showing dead bodies and weapons, communications equipment and money it said was captured from rebels.

The United States also declared Sunday that it would "hold accountable" any Syrian official involved in the release or use of the country's chemical weapons.

Fears have been rising in the West after reports that Assad might be prepared to use his arsenal of chemical weapons to save his embattled regime.

According to The Journal, Washington has also tried to keep ships believed to carry arms and fuel for Syria from passing through the Suez Canal.

One ship, the Amin, currently seeking permission to enter the canal is owned by a subsidiary of Islamic Republic of Iran Shipping Lines, the report said.

US officials are holding talks with the Egyptian government in an effort to block the Amin's passage, arguing that it isn't properly flagged and doesn't have internationally recognized insurance, the paper noted.

It added that the United States also has been providing intelligence about  Syria to the Turkish and Jordanian militaries working closely with the rebels.

This information includes imagery from military satellites and other surveillance equipment that provides details about Syrian military sites that could be helpful to the rebels, The Journal pointed out. .

US officials acknowledge that despite their efforts, some shipments of arms and fuel for Syria have slipped through.