Some, "unaware of the meaning of resistance, imagine that when the resistance becomes weak, Islamic Iran will also become weak... Iran is strong and powerful and will become even more powerful," Khamenei said in his first speech after Assad's fall.

Syrian insurgents' lightning push to Damascus from their strongholds in the northwest ended the decades-long rule of Assad's family, once an ally of Tehran.

Assad had long played a strategic role in Iran's anti-Israel "axis of resistance".

The axis of resistance includes Hezbollah, as well as Hamas in Gaza, Houthi rebels in Yemen and smaller Shia militia groups in Iraq.

All of the groups are united in their opposition to Israel and its occupation of the Palestinian territories.

The Iranian supreme leader, who has the final say in his country's affairs, accused the United States and "the Zionist regime", Israel, of plotting against Assad.

"No one should doubt that what has taken place in Syria is the product of a joint US-Israeli plot," he said.

He also blamed another "neighboring state of Syria" for its "obvious role" in the recent developments, without naming the country.

Iraq, Israel, Jordan, Lebanon and Turkey all share borders with Syria.

Of those neighbors, Turkey has long supported the ouster of Assad.

According to Khamenei, different "invaders" in Syria were pursuing different aims.

"Their goals are different, some of them are seeking to seize the lands of northern or southern Syria, America is seeking to strengthen its position in the region," he said.

Turkey has forces in northern Syria, while in the south the Israeli army has sent troops into a UN-patrolled buffer zone on the countries' shared border east of the Israeli-occupied Golan Heights.

The US also has troops based in Syria, where they have worked with Kurdish-led fighters battling the Islamic State group.