WASHINGTON — The United States will provide its own justification for military intervention in Syria if necessary, but any response to the Assad regime’s alleged use of chemical weapons will be limited, the White House said on Thursday.

“When the president reaches a determination about the appropriate response … and a legal justification is required to substantiate or to back up that decision, we’ll produce one on our own,” White House spokesman Josh Earnest said Thursday at a press briefing.

“What we’re talking about here is something very discrete and limited,” he added.

Meanwhile, a United Nations Security Council of the five permanent members ended Thursday evening with no real progress over an agreement on how to respond to last Wednesday’s killing of hundreds of Syrian civilians in an alleged chemical weapons attack.

The meeting Thursday afternoon started breaking up after less than an hour, with the ambassadors of China, France, Britain, Russia and the United States steadily walking out. The meeting was reportedly being held at the behest of Russia.

It was the second time in two days that the five Security Council powers came out of a meeting on Syria with no progress.

Russia remains firmly opposed to any action, saying there is no evidence the Syrian regime was responsible for the attack, as the US and its allies contend.

During the press briefing in Washington, Earnest rejected comparisons made by members of Congress and commentators alike that drew parallels between imminent military action in Syria and the 2003 US invasion of Iraq. Then-state senator Barack Obama opposed President George W. Bush’s involvement in Iraq, which was supported by claims that Iraqi leader Saddam Hussein was stockpiling weapons of mass destruction.

“I don’t agree these are similar situations. There are very important differences,” said Earnest. “What we saw in that circumstance that an administration was searching high and low to produce evidence to justify a military invasion, an open-ended military invasion of another country, with the final goal being regime change,” he said. “That was the articulated policy of the previous administration.”

Obama administration members, including the president himself, have emphasized repeatedly that the US goal is primarily to issue a response to the chemical weapons attack, and not to directly bring about a regime change in Damascus.

Later on Thursday, a top-level group of administration representatives led by US Secretary of State John Kerry and Secretary of Defense Chuck Hagel were to brief House and Senate leaders on the situation in Syria. The briefing — in which the cabinet members were to be joined by National Security Adviser Susan Rice and Director of National Intelligence James Clapper — was scheduled after a number of key senators voiced their reservations about the Obama administration’s policy toward Damascus.

Kerry, Secretary of Defense Chuck Hagel and senior White House officials were convene a briefing, set for 6 p.m. ET, for leaders of the House and Senate from both parties, as well as the chairmen and ranking members of relevant congressional committees.

Congressional leaders from both parties have repeatedly said in recent days that Obama must consult Congress prior to initiating any military involvement in Syria, including airstrikes. Over one quarter of House representatives — 98 Republicans and 18 Democrats — signed a letter demanding that any military action in Syria come first to a vote in Congress.

“I don’t want to leave you with the impression that this conference call is the first or the last medium for consulting congress. It’s not,” Earnest emphasized Thursday afternoon.

Just hours before the scheduled conference call, British Prime Minister David Cameron defended intervention in Syria during a raucous session of Parliament.

The flurry of legislative consultations, combined with an increasingly long time frame for UN investigators of last week’s alleged chemical weapons attack against Syrian civilians, mean that any retaliatory strikes against the Syrian regime are likely to be days away.

AP contributed to this report.