WASHINGTON (Reuters) - President Barack Obama vowed on Wednesday that the Syrian government would face “international consequences” for last week’s deadly chemical attack, but made clear any military response would be limited to avoid dragging the United States into another war in the Middle East.

Casting the need for action based on U.S. national security interests instead of humanitarian grounds, Obama made his case to a war-weary American public for what is looking like an all-but-certain use of force in Syria, where he has long been reluctant to intervene.

While saying he and allied leaders had not yet made a decision on military strikes against Syrian President Bashar al-Assad’s loyalists, he left little doubt that the choice was not whether to act but when.

“We have concluded that the Syrian government in fact carried these out, and if that’s so, then there need to be international consequences,” Obama told “PBS Newshour” in a televised interview.

There were growing signs, however, that the timeline for launching any military strike on Syria could be complicated not only by the U.N. weapons inspectors’ continued presence there but by the Obama administration’s efforts to coordinate with international partners and growing demands for consultation with U.S. lawmakers.

On top of that, Britain - a key player in any air assault on Syria - changed its stance on Wednesday, saying the U.N. Security Council should see findings from weapons inspectors before any military action is taken and that the British parliament should vote on the matter twice.

For his part, Obama insisted that while Assad’s government must be punished, he intended to avoid repeating Washington’s errors from the Iraq war.

“I have no interest in any open-ended conflict in Syria, but we do have to make sure that when countries break international norms on weapons like chemical weapons that could threaten us, that they are held accountable,” Obama said.

It was Obama’s clearest justification yet for a tough response against Assad, who is accused of having crossed a “red line” for large-scale chemical weapons use that Obama established just over a year ago. Hundreds of people were killed in a poison gas attack on Damascus suburb last Wednesday.

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SYRIANS PREPARE FOR ATTACK

In Damascus on Wednesday, people left homes close to potential targets as U.S. officials sketched out plans for multi-national air strikes on Syria that could last for days. U.N. chemical weapons experts completed a second field trip to rebel-held suburbs searching for evidence.

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But as U.N. chief Ban Ki-moon appealed for unity among world powers and sought more time for the inspectors to complete their work, Washington and its European and Middle East allies said their minds were made up and that Assad must face retribution for using banned weapons against his people.

Syria’s government, supported notably by its main arms supplier Russia, cried foul. It blamed rebel “terrorists” for releasing the toxins with the help of the United States, Britain and France, and warned it would be a “graveyard of invaders.”

Syrian officials say the West is playing into the hands of its al Qaeda enemies. The presence of Islamist militants among the rebels has deterred Western powers from arming Assad’s foes. But the West says it must now act to stop the use of poison gas.

Britain pushed the other four veto-holding members of the U.N. Security Council at a meeting in New York to authorize military action against Assad to protect Syrian civilians - a move certain to be blocked by Russia and, probably, China. The meeting ended without a decision.

The United States and its allies say a U.N. veto will not stop them. Western diplomats called the proposed resolution a maneuver to isolate Moscow and rally a coalition behind air strikes. Arab states, NATO and Turkey also condemned Assad.

But British Prime Minister David Cameron was forced on Wednesday to push back his timetable after coming under fierce domestic and international pressure, and it was unclear how that might affect any Syria attack plans.

Just a day after recalling Britain’s parliament to vote on how to respond to Syria’s suspected use of chemical weapons, Cameron was ambushed when the opposition Labour party said it wanted greater parliamentary scrutiny and rebel lawmakers in his own ruling Conservative Party said they would oppose him.