WASHINGTON — (AP) — President-elect Donald Trump said Saturday that the U.S. military should stay out of the fast-escalating conflict in Syria, where a dramatic rebel offensive reached the capital and threatened the rule of Syria's Russian- and Iranian-allied president. "THIS IS NOT OUR FIGHT," Trump declared on social media.

As world leaders watched the stunning rebel advance, with its potential to alter the balance of power in the Middle East, President Joe Biden's national security adviser separately stressed that the Biden administration had no intention of intervening.

“The United States is not going to ... militarily dive into the middle of a Syrian civil war," Jake Sullivan told an audience in California.

Sullivan said the U.S. would keep acting as necessary to keep the Islamic State — a violently anti-Western extremist group not known to be involved in the offensive but with sleeper cells in Syria's deserts — from exploiting openings presented by the fighting.

Insurgents’ stunning march across Syria appeared to reach its goal hours after both men spoke, with rebels entering Damascus after claiming many of the country's other major cities within roughly 10 days. The head of a Syrian opposition war monitor said early Sunday that Assad left the country for an undisclosed location.

Trump's comments on the dramatic rebel push were his first since Syrian rebels launched their advance late last month. They came while he was in Paris for the reopening of the Notre Dame cathedral.

In his post, Trump said Assad did not deserve U.S. support to stay in power.

Assad's government has been propped up by the Russian and Iranian military, along with Hezbollah and other Iranian-allied militias, in a now 13-year-old war against opposition groups seeking his overthrow. The war, which began as a mostly peaceful uprising in 2011 against the Assad family's rule, has killed a half-million people, fractured Syria and drawn in a more than a half-dozen foreign militaries and militias. The U.S. early on closed its embassy in Syria and imposed sanctions over the brutality of Assad's conduct of the war.

The insurgents are led by Hayat Tahrir al-Sham, which the U.S. has designated as a terrorist group and says has links to al-Qaida, although the group has since broken ties with al-Qaida.

The insurgents met little resistance so far from the Syrian army, the Russian and Iranian militaries or allied militias in the country.

The Biden administration said the ease of Syrian opposition forces' capture of government-held cities demonstrates how Russia's war in Ukraine and Iran's and Iranian militias' fight against Israel in Gaza and Lebanon have diminished them.

“Assad’s backers — Iran, Russia and Hezbollah — have all been weakened and distracted," Sullivan said Saturday at an annual gathering of national security officials, defense companies and lawmakers at the Ronald Reagan Presidential Library in Simi Valley.

“None of them are prepared to provide the kind of support to Assad that they provided in the past,” he later added.

The U.S. has about 900 troops in Syria, including U.S. forces working with Kurdish allies in the opposition-held northeast to prevent any resurgence of the Islamic State group.

Gen. Bryan Fenton, head of U.S. Special Operations Command, said he would not want to speculate on how the upheaval in Syria would affect the U.S. military’s footprint in the country. “It’s still too early to tell,” he said.

What would not change is the focus on disrupting IS operations in Syria and protecting U.S. troops, Fenton said during a panel at the Reagan event.

Syrian opposition activists and regional officials have been watching closely for any indication from the incoming Trump administration on how the U.S. would respond to the rebel advances against Assad.

Robert Wilkie, Trump's defense transition chief and a former secretary of the Department of Veterans Affairs, said during the same California event that the collapse of the “murderous Assad regime” would be a major blow to Iran's power.

In his post, Trump said Russia “is so tied up in Ukraine” that it “seems incapable of stopping this literal march through Syria, a country they have protected for years.” He said rebels could possibly force Assad from power.

The president-elect condemned the overall U.S. handling of the war but said the routing of Assad and Russian forces might be for the best.

“Syria is a mess, but is not our friend, & THE UNITED STATES SHOULD HAVE NOTHING TO DO WITH IT. THIS IS NOT OUR FIGHT. LET IT PLAY OUT. DO NOT GET INVOLVED!” he wrote in Saturday's post.

An influential Syrian opposition activist in Washington, Mouaz Moustafa, interrupted a briefing to reporters to read Trump’s post and appeared to choke up. He said Trump’s declaration that the U.S. should stay out of the fight was the best outcome that the the Syrians aligned against Assad could hope for.

Rebels have been freeing political detainees of the Assad government from government prisons as they advance across Syria. Moustafa pledged to reporters Saturday that opposition forces would be alert for any U.S. detainees among them and do their best to protect them.

Moustafa said that includes Austin Tice, an American journalist missing for more than a decade and suspected to be held by Assad.

Hayat Tahrir al-Sham renounced al-Qaida in 2016 and has worked to rebrand itself, including cracking down on some Islamic extremist groups and fighters in its territory and portraying itself as a protector of Christians and other religious minorities.

While the U.S. and United Nations still designate it as a terrorist organization, Trump's first administration told lawmakers that the U.S. was no longer targeting the group's leader, Abu Mohammed al-Golani.

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Copp reported from Simi Valley, California.

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