The jihadi group surging through Iraq and Syria is using large captured US-made weapons and has access to anti-tank rockets supplied by Saudi Arabia to a moderate rebel group, according to a report published on Monday.

The study by the London-based Conflict Armament Research consultancy found that Islamic State (Isis) militants had access to large numbers of US weapons, which they were shifting to key battlefields.

The report drew no conclusions about how the weapons were sourced. However, the capitulation of the Iraqi army in northern Iraq on 10 June gave the jihadis access to military arsenals in the north of the country, which were full of US-supplied assault rifles and ammunition, as well as heavy weapons.

The report was compiled from a list of weapons captured from Isis by Kurdish militias over a 10-day period in July.

Of most interest was the capture of two M-79 rockets that were identical to a batch of such weapons supplied by Saudi Arabia to rebels in southern Syria in January 2013. The rockets had been sourced from a Croatian arms supplier and ferried to anti-Assad fighters who were identified as non-jihadis.

The potential for weapons provided to one group to fall into the hands of other militants fighting in Syria has often been cited by the US and others as a reason not to heavily arm groups involved in the civil war.

Calls by rebels for heavy weapons that would allow them to confront Syria's air force or tanks have regularly been turned down. But with the war zone now flush with such weapons – largely in the hands of jihadis who oppose both mainstream rebels and the Syrian regime, Washington is rethinking its involvement.

Moderate rebels in northern Syria have in the past fortnight confronted Isis forces who arrived in Humvees and armoured troop carriers supplied by the US to Iraq. The rival rebel groups found it difficult to stop Isis with the weapons they had.

The vehicles were seized en masse from bases near Mosul and driven across the Syrian border, where they have revitalised a battlefield that had become a three-way stalemate in eastern and northern Syria.

US jets have in the past six weeks bombed Humvees and US-made troop carriers in northern Iraq. Many were used by the US military during the nearly nine-year war and occupation, then handed over by US officers as they left Iraq. Others were bought directly from Washington.

Using such vehicles, Isis fended off assaults on Sunday in far the western Anbar province by Iraqi forces, backed by fighter jets. US jets again attacked Isis positions near the Haditha dam, the second one of Iraq's two most important waterways, which has become a key target for jihadis.

Isis fighters drive a US-made Humvee in Syria. The militants have so much heavy weapons and armoured transport that rival rebels cannot confront them. Photograph: Reuters

The publication of the arms report came as the White House was finalising its plan on Monday for a sustained confrontation with Isis that could involve extending air strikes against the jihadis in Iraq to their strongholds in Syria.

Barack Obama will use a key address to the nation on Wednesday – the eve of the 9/11 anniversary – to lay out a detailed strategy for his stated plan to "degrade and destroy" the jihadis.

In advance of the speech, which could mark a significant shift in his administration's foreign policy, officials have been trying to galvanise support for that strategy at home and abroad.

On Tuesday, the president will meet congressional leaders in Washington, where both Democrats and Republicans are open to intensifying the fight against Isis. Hawks from both parties are urging the White House to attack Isis in regions of Syria where it has solidified its power base.

However, less than before Obama was due to lay out his plan to Congress, administration officials are believed to be undecided about the wisdom of switching the focus of the attack to Syria. The decision is expected to depend in part on the response from other governments to Wednesday's speech.

The state department has dispatched diplomats across the Middle East, convinced that expanding the fight against Sunni extremists can only succeed with the backing of regional powers such as Saudi Arabia, Qatar and the United Arab Emirates.

The Arab League on Monday agreed in a resolution to combat Isis extremists, but the statement from the 22-member body was vague and stopped short of explicitly backing American military action.

After last week's Nato summit in Wales, the White House calculated it has enough support from key allies to build an international coalition that will support its action against Isis, even though officials acknowledge they face greater resistance from the region.

The US defence secretary, Chuck Hagel, said on a visit to Georgia at the weekend: "This is a galvanising moment for Nato and our partners."

Obama's speech on Wednesday will be a pivotal moment in his presidency. His administration has struggled to recover from his recent admission that "we don't have a strategy yet" for fighting Isis, which has taken over swaths of Iraq and Syria in a conflict that has become a magnet for militant fighters from across the world.

Since then, Obama has adopted an unusually aggressive stance against Isis, which his top intelligence advisers believe could overtake al-Qaida as the west's most dangerous enemy. Previewing his national address on Sunday, Obama told NBC's Meet The Press he was "preparing the country" for confronting Isis.

"The next phase is now to start going on some offence," he said. "But this is not going to be an announcement about US ground troops. This is not the equivalent of the Iraq war. What this is is similar to the kinds of counter-terrorism campaigns that we've been engaging in consistently over the last five, six, seven years."

The White House believes it has the power to continue strikes in Iraq and possibly expand air strikes into Syria without congressional authorisation but Obama is determined to secure an endorsement from lawmakers. "I do think it's important for Congress to understand what the plan is, to have buy in, to debate it," he said.