Saudi Arabia is preparing to spend millions of dollars to arm and train thousands of Syrian fighters in a new national rebel force to help defeat Bashar al-Assad and act as a counterweight to increasingly powerful jihadi organisations.

Syrian, Arab and western sources say the intensifying Saudi effort is focused on Jaysh al-Islam (the Army of Islam or JAI), created in late September by a union of 43 Syrian groups. It is being billed as a significant new player on the fragmented rebel scene.

The force excludes al-Qaida affiliates such as the Islamic State in Iraq and al-Sham and Jabhat al-Nusra, but embraces more non-jihadi Islamist and Salafi units.

According to one unconfirmed report the JAI will be trained with Pakistani help, and estimates of its likely strength range from 5,000 to more than 50,000. But diplomats and experts warned on Thursday that there are serious doubts about its prospects as well as fears of "blowback" by extremists returning from Syria.

The Saudi intelligence chief, Prince Bandar bin Sultan, is also pressing the US to drop its objections to supplying anti-aircraft and anti-tank missiles to the JAI. Jordan is being urged to allow its territory to be used as a supply route into neighbouring Syria.

In return, diplomats say, Riyadh is encouraging the JAI to accept the authority of the US and western-backed Supreme Military Council, led by Salim Idriss, and the Syrian Opposition Coalition.

"There are two wars in Syria," said Mustafa Alani, an analyst for the Saudi-backed Gulf Research Centre. "One against the Syrian regime and one against al-Qaida. Saudi Arabia is fighting both."

Saudi Arabia has long called publicly for arming the anti-Assad rebels and has bridled at US caution. It has been playing a more assertive role since September's US-Russian agreement on chemical weapons - which it saw as sparing the Syrian leader from US-led air strikes and granting him a degree of international rehabilitation.

The JAI is led by Zahran Alloush, a Salafi and formerly head of Liwa al-Islam, one of the most effective rebel fighting forces in the Damascus area. Alloush recently held talks with Bandar along with Saudi businessmen who are financing individual rebel brigades under the JAI's banner. Other discreet coordinating meetings in Turkey have involved the Qatari foreign minister, Khaled al-Attiyeh, and the US envoy to Syria, Robert Ford.

In one indication of its growing confidence – and resources – the JAI this week advertised online for experienced media professionals to promote its cause.

The appearance of an "Army of Muhammad" – with its equally obvious Islamic resonance – appears to be part of the same or related effort proposed by Syrian Sunni clerics to unite disparate rebel groups into a 100,000-strong force by March 2015.

It is too early, however, to see any impact of the Saudi move on the ground. "Militarily it's not significant," said one senior western official.

"I don't see it producing any dramatic change yet. It's a political step. These new rebel formations seem to be relabelling themselves and creating new leadership structures. It's part of a quite parochial political game – and above all a competition for resources."

But the Saudis are making an energetic case for their strategy – and playing on western anxieties. "The Saudis are saying that if you don't join the fight against Assad you will end up with a much bigger jihadi problem," said Emile Hokayem of the International Institute for Strategic Studies. "They are being a lot more proactive. That means taking the rebellion a lot more seriously and trying to develop as many proxies and allies as possible."

Saudi assertiveness has grown along with unhappiness over US policy towards Syria and Iran, the kingdom's regional rival. Prince Turki al-Faisal, the former Saudi intelligence chief, described Obama's approach to Syria as "lamentable".

Last month the Saudis cancelled their annual speech at the UN general assembly and turned down their first election to a security council seat in protest over the Syrian situation. The Saudis, like the Israelis, also fear a US "grand bargain" that leaves Iran free to develop nuclear weapons.

Alani, echoing official Saudi views, warned of the risk from an emboldened al-Qaida unless more moderate forces prevailed in Syria. "Al-Qaida is getting stronger," he said. "It is undermining the Syrian revolution and giving the US an argument for not supporting it. It will backfire against Saudi Arabia and the Gulf sooner or later – like what happened in Afghanistan, Pakistan and Iraq."

Other experts argue that the kingdom is taking risks by being so proactive, relying on funding and weapons for influence, concentrating on military pressure on Assad without developing a clear political strategy and focusing on strengthening groups with an overtly Sunni character.

"The Saudi leadership should be careful what it creates in Syria," Yezid Sayegh of the Carnegie Foundation warned in a recent commentary. "Muhammad's Army may eventually come home to Mecca."

The effort also faces problems of capacity, coordination and delivery. "The Saudis and Qataris lack the means to shape insurgent groups," suggested Thomas Pierret of Edinburgh University.

"They have a lot of money but very poor intelligence and human resources and organisational skills. They are very dependent on the western military. They are too used to having relationships with clients and using personal networks.

"That's why they've been forced to turn to Syrian groups which already have military credibility. They are becoming less selective and more realistic and putting aside their reservations about who they support. But I doubt they are able to unify the whole thing. The Saudis say 'you should unite and we will give you money.' But some will end up getting more money than others and the coalition will break apart."