Mideast powers opposed to President Bashar Assad have dramatically stepped up weapons supplies to Syrian rebels in coordination with the U.S. in preparation for a push on the capital of Damascus, officials and Western military experts said Wednesday.

Four sources, including an Arab official, a diplomat and a military expert, described a system in which Saudi Arabia and Qatar provide the funding for the weapons. Jordan and Turkey provide the land channels for the shipments to reach the rebels, while all coordinate with the U.S. and other Western governments on the shipments’ destinations.

All must agree for a shipment to go through. The Arab official said some of the arms are being purchased from Croatia, or from U.S. draw-downs in unspecified European countries. He said other sources were black market arms dealers across Europe and the Mideast.

A carefully prepared covert operation is arming rebels, involving Jordan, Saudi Arabia, Turkey and Qatar, with the United States and other Western governments consulting, and all parties hold veto power over where the shipments are directed, according to a senior Arab official whose government is participating. His account was corroborated by a diplomat and two military experts.

The Arab official said the number of arms airlifts has doubled in the past four weeks. He did not provide exact figures on the flights or the size of the cargo. Jordan opened up as a new route for the weapons late last year, amid U.S. worries that arms from Turkey were going to Islamic militants, all four told The Associated Press in separate interviews. Jordan denies helping funnel weapons to the rebels.

The two military experts, who closely follow the traffic, said the weapons include more powerful, Croatian-made anti-tank guns and rockets than the rebels have had before.

The Arab official said there was a “master plan” for the rebels to seize Damascus. He and the diplomat spoke to the AP on condition that their identities and their nationalities not be disclosed because the operation was covert.

“The idea is that the rebels now have the necessary means to advance from different fronts — north from Turkey and south from Jordan — to close in on Damascus to unseat Assad,” the Arab official said. He declined to provide details, but said the plan is being prepared in stages and will take “days or weeks” for results.

Rebels have captured suburbs around Damascus but have been largely unable to break into the heavily guarded capital. Instead, they have hit central neighborhoods of the city with increasingly heavy mortar volleys from their positions to the northeast and south.

The sources said the material was destined for “secular” fighters not necessarily linked to the Free Syrian Army, the nominal umbrella group for the rebels. However, Islamist groups, such as Jabhat al-Nusra, have emerged as the most powerful factions in the opposition, imposing Islamic Sharia in many rebel-held areas. The Syrian government and its allies have repeatedly accused Gulf countries of arming the extremists.