WASHINGTON — Conceding that a peaceful resolution in Syria now appears remote, President Obama warned Monday for the first time that use or movement of chemical or biological weapons by forces loyal to President Bashar Assad would constitute a "red line" for U.S. military intervention.

Obama acknowledged his frustration that diplomacy has done little to protect civilians or stem the growing bloodshed in the 17-month conflict. International efforts to persuade Assad to step down, to negotiate an effective cease-fire or to facilitate a political transition have failed so far.

"At this point, the likelihood of a soft landing seems pretty distant," Obama said in a brief, unscheduled news conference at the White House.

Obama spoke as activists in Syria reported at least 127 people killed Monday in urban street battles and other clashes between armed rebels and government forces. Activists also described a grisly discovery of at least 10 bodies that showed signs of torture in the Qaboun area of Damascus, the capital.

The violence escalated after a one-day lull on Sunday, the start of the three-day Eid al-Fitr holiday that marks the end of the holy month of Ramadan.

Rights groups said the latest assaults by tanks and warplanes caused two houses to collapse in Aleppo, Syria's largest city, killing at least 14 people. The buildings were in the Al-Sakhour and Qadi Askar neighborhoods, said activist Mohammed Saeed, reached by Skype inside the city. The reports could not be independently confirmed.

Among the dead in Aleppo was a Japanese journalist; a video posted online showed her body in a field hospital.

Obama said he has not ordered the U.S. military into action. But he said his administration has "put together a range of contingency plans," including a response if it appears Assad's forces are preparing to use poison gas or biological weapons.

"We have communicated in no uncertain terms with every player in the region that that's a red line for us, and that there would be enormous consequences if we start seeing movement on the chemical weapons front, or the use of chemical weapons," Obama said. "That would change my calculations significantly."

The administration has provided communications gear and other nonlethal aid to the rebels, but it has rejected calls to arm the main rebel force, the Free Syrian Army, or to set up a no-fly zone to prevent Syrian warplanes from attacking civilian areas. U.S. officials say they are cautious, in part, because the ragtag coalition of militias includes some aligned with al-Qaida and other jihadist groups.

Obama said the danger from Syria's weapons stockpiles "concerns our close allies in the region, including Israel. It concerns us. We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people."

Syria is believed to have huge stockpiles of sarin nerve agent, mustard blister gas and cyanide.

U.S. officials believe that Assad's government retains control of its weapons depots, but Obama offered a cautious response when asked if he thought that the stockpiles are safe.

"In a situation this volatile, I wouldn't say that I'm absolutely confident," he said. "What I'm saying is we're monitoring that situation very carefully."

Concern about the weapons began to rise in mid-July, when Israeli and U.S. officials reported that Syria might be moving some of the materials, possibly in preparation for their use or to safeguard them from advancing rebel fighters.

Israeli officials warned that they would not allow the weapons to fall into the hands of adversaries such as Hezbollah, the Syrian-backed Lebanese militant group.

U.S. officials hope to avoid any Israeli intervention, which could further inflame Syria's civil war.

Information from The Associated Press is included in this report.