Iran’s supreme leader has claimed the US and Israel acted as the command centre that engineered the downfall of Syria’s former president, Bashar al-Assad, and the ousting of Iran from the country.

He also claimed Iranian intelligence had sent warnings to the Assad government about a possible attack for three months and predicted Syrian youth will eventually recapture the country, remarks that are unlikely to make it easier for Iran to build strong relations with the new leaders in Damascus.

Making his first comments since the collapse of a key pillar of Iran’s “axis of resistance”, Ayatollah Ali Khamenei told a gathering in Tehran: “There should be no doubt that what happened in Syria is the product of a joint American and Zionist plan. Yes, a neighbouring government in Syria plays an obvious role in this regard, and is still playing it – everyone sees this – but the main conspirator, mastermind, and command centre are in America and the Zionist regime. We have evidence. This evidence leaves no room for doubt.”

His reference to a neighbouring government seems to be about Turkey, which played a key role in supporting the opposition forces that fanned out from Idlib in a well-planned military operation that was far more successful than they had ever envisaged.

But Khamenei insisted: “Let everyone know that this situation will not remain as it is. The fact that some people in Damascus celebrate, dance, and violate the homes of others while the Zionist regime bombs Syria, enters their territory with tanks and artillery, is unacceptable. The Syrian youth will undoubtedly stand firm and overcome this situation.”

He also called for the media to be be less negative about Iran, reflecting the fact that in recent days an open and self-critical inquest has been under way in Iran about what went wrong with its strategy in Syria, and its consequences for Iran’s future security, including whether to seek reconciliation with the west or instead build nuclear weapons. There have already been two closed sessions of the parliament to discuss the crisis, one held with the foreign minister and the other with the military.

Iran is already losing privileged access to the new Damascus government due to be dominated by Hayat Tahrir al-Sham (HTS) until at least next March, and was not among the group of Arab states offered a briefing on Tuesday in the capital. By contrast, Qatar, a country that refused to recognise Assad, is due to reopen its embassy shortly, and is likely to be a key player in trying to persuade the Sunni HTS it needs to be inclusive if it is to thrive and enjoy sanctions relief. In a further blow to Iran’s status as defenders of the Palestinian cause, Hamas, the leaders of the Palestinian resistance in Gaza, has welcomed the fall of Iran’s ally Assad.

One of Shia Iran’s earliest concerns is protection of Shia mosques such as the Sayyida Zaynab mosque near Damascus.

Across Iranian social media, and in the reformist press, an open debate has started about how much Iran invested in propping up Syria and whether this was worthwhile. An early discussion point is whether Iran will ever be able to persuade the Syrian government to repay the financial debt owed to Iran, variously put at $30bn to $40bn. Investments in car plants are also in jeopardy. So far 10,000 Iranians have been flown home from Syria.

Pictures of the Syrian prisons such as Sednaya are also widely available on Iranian X.

Faizullah Arabsorkhi, a reformist political activist, wrote: “If Bashar Assad had believed that governing the country without the people was impossible, he might have had a different fate.”

Javad Zarif, Iran’s vice-president for strategic affairs, admitted mistakes had been made in Syria but argued the moment had come to change Iran’s foreign policy mindset from a threat-oriented attitude to an opportunity-oriented one.

He said: “The reason for Assad’s downfall was his arrogance over his victory over the militants, and his failure to convert the military victory into a win-win political agreement and the establishment of an inclusive government. But the speed of the fall was unpredictable for no one.

“Military victories are temporary, and history shows that if they do not lead to a win-win political agreement, they can be the beginning of defeat.”

He insisted the axis of resistance would not be weakened, but many observers have pointed to the loss of ammunition supply lines into Hezbollah in Lebanon.

Zarif and the foreign minister, Abbas Araghchi, have been trying to end Iran’s regional isolation by strengthening diplomatic relations with countries such as Saudi Arabia. They are also trying to reopen talks with the west on the monitoring of its nuclear programme.

A persistent Iranian analysis of the surprise speed of the collapse was the impact of sanctions since the US Caesar Act implemented in June 2020, and the fact that soldiers were being paid only $16 to $17 a month, sufficient for 3 days. Araghchi said the army had seemed to be psychologically unprepared to fight.

One of Iran’s most cited political analysts, Abbas Abdiu, delivered a scathing note saying “in just one week all political, economic, and military investments have gone up in smoke”. He blamed those who had refused to listen to the criticism of the strategy, and insisted official state TV channels refuse to give airtime that warned the defence of Assad would come unstuck.