WASHINGTON — President Obama warned Syria on Monday that it would face American military intervention if there were signs that its arsenal of unconventional weapons was being moved or prepared for use. It was Mr. Obama’s first direct threat of force against Syria, as he has resisted being drawn into the bloody 18-month rebellion.

The president’s warning raises the pressure on President Bashar al-Assad, whom Mr. Obama again called on to relinquish power. And it underscores the deepening alarm among American officials that, as Syria sinks further into civil war, its unconventional weapons could be seized by radical forces tied to terrorist groups like Hezbollah or Al Qaeda.

The warning brings Mr. Obama, who has brushed aside calls to impose a no-fly zone or to arm the Syrian rebels, a step closer to direct American engagement. The specter of unconventional weapons being loosed in the heart of the Arab world, he said, would upend his calculation that military intervention would only worsen the situation.

“We cannot have a situation in which chemical or biological weapons are falling into the hands of the wrong people,” Mr. Obama said in response to questions at an impromptu news conference at the White House. “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 weapons moving around or being utilized.”