WASHINGTON — President Barack Obama on Monday for the first time threatened U.S. military intervention in Syria’s civil war, warning the beleaguered regime of President Bashar Assad against breaching a U.S. “red line” of moving or using chemical or biological weapons.

Addressing a news conference, Obama indicated that his administration believes that Assad could resort to chemical or biological weapons to stave off his overthrow and that Islamic extremists allied with the rebels could acquire such weapons if they were deployed.

“That’s an issue that doesn’t just concern Syria. It concerns our close allies in the region, including Israel. It concerns us,” said Obama. “We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people.”

He added: “We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized. That would change my calculus. That would change my equation.”

Obama has joined other Western and Arab leaders in calling for Assad to relinquish power. He also has authorized non-lethal aid to the loose coalition of rebel factions, such as radios, and some $82 million in U.S. humanitarian aid for tens of thousands of civilians displaced by the war.

But he has resisted becoming more deeply involved in the 17-month-old conflict that has killed an estimated 20,000 people, embroiled Damascus and other cities in savage fighting and set the majority Sunni Muslims against the dominant minority Alawite sect, an offshoot of Shiite Islam.

Obama said that while he has “not ordered military engagement in the situation,” U.S. policy would change with the movement or use of Syria’s chemical or biological weapons, which are believed to include mustard gas and nerve agents like sarin or VX.

“We are monitoring that situation very carefully. We have put together a range of contingency plans,” Obama said. “We have communicated in no uncertain terms with every player in the region that that’s a red line for us and there would be enormous consequences if we start seeing movement on the chemical weapons front or the use of chemical weapons.”

Obama didn’t elaborate on the “players” to whom he was referring. But he appeared to be referring to Syria’s key foreign supporters, Iran and Hezbollah, the Shiite Muslim militia that dominates neighboring Lebanon, as well as to Syrian rebels, their Sunni Muslim patrons led by Saudi Arabia and Qatar, and to Islamic extremists linked to al Qaida.

Obama’s comments were greeted with derision by Syrian activists on the social-networking sites Facebook and Twitter. They accused him of threatening intervention only when Israel was at risk.

One Twitter user compared Obama to Russian President Vladimir Putin, one of the Syrian regime’s few foreign allies: “Both blabber about ‘red lines,’ have kept Assad afloat in blood-soaked power.” Another tweet, from a user called SyriaTime, said the president’s warning so late in the crisis is akin to saying, “Sure, genocide is fine.”

For months, Syrian activists – in exile and inside the country – have called for some kind of international help in their fight to dislodge the Assad dynasty and its powerful military force. Debates rage over the form of the intervention, ranging from the imposition of a no-fly zone to direct strikes on regime targets.

A poll released this month by the International Republican Institute, a Washington-based democracy-promotion group, found that a majority of the anti-Assad opposition inside Syria back outside intervention, with their first choice a no-fly zone.

The close second and third options were the establishment of so-called “safe zones” for the opposition – the rebels already have carved out de facto corridors – and supplying weapons to the rebels.

The institute noted that the results came from unconventional polling methods – such as using known activists instead of a random sampling – because the widespread violence makes on-the-ground polling impossible.

“Respondents exhibited support for a range of international armed intervention measures, with the most support going to actions that would not involve an international presence on the ground,” the introduction to the survey stated.