The United States refused to set new "red lines" for Syria Wednesday as claims of a horrific chemical arms strike reignited debate over Washington's reluctance to intervene in the civil war.

Claims by Syrian rebels that President Bashar al-Assad's forces massacred 1,300 people in a chemical weapons attack near Damascus sparked new pressure for action on Capitol Hill.

But the top US military officer, General Martin Dempsey, pushed back on calls for air strikes, which he said could embroil America in an open-ended war.

The new claim of chemical warfare lured Barack Obama's administration onto difficult political ground after the president was accused earlier this year of not enforcing "red lines" he set over such attacks.

"I'm not talking about red lines. I'm not having a debate or conversation about red lines ... I'm not setting red lines," said State Department spokeswoman Jen Psaki.

The White House meanwhile refused to specify what, if any action Assad would face if it was proven that his forces were behind the attack in which many victims apparently choked to death.

Spokesman Josh Earnest focused instead on a team of United Nations inspectors in Syria to probe previous claims of chemical attacks, saying they should be allowed to examine the new alleged incident.

"Before we suggest what may or may not happen as a result of the investigation's findings being revealed, let's start with making sure that this investigation actually gets conducted in a manner that is credible," he said.

A year ago, Obama said at a White House press conference that any use of Assad's deadly chemical arsenal would cross a "red line" and would entail "enormous consequences."

Many observers interpreted those remarks as a sign Obama would contemplate direct military action against Syrian government forces.

Then, Washington concluded in June that Assad's forces had indeed used chemical arms, including the nerve gas sarin, in attacks that killed up to 150 people.

In response, Washington promised to significantly toughen its stance on Syria and said it would provide military support to rebels for the first time.

But it has refused to specify exactly what it is doing, as the information is classified, and much of the assistance is believed to have yet to reach vetted opposition groups.

Obama's opponents on Capitol Hill accuse him of damaging his own credibility by refusing to back up infringed red lines with overt military action.

The latest reports represent another Middle Eastern headache for a White House currently also struggling to frame a coherent response to a coup in Egypt.

Eliot Engel, the top Democrat on the House Foreign Affairs committee, said that if the report was credible, "a red line has been crossed again."

"The US has two options: continue to largely stand on the sidelines as the regime slaughters its own people, or tip the balance of power against a brutal dictator by degrading its ability to attack civilians."

Republican Senator John McCain, a critic of Obama's refusal to use military force to protect civilians in Syria, weighed in on Twitter.

"No consequence for Assad using chemical weapons & crossing red line -- we shouldn't be surprised he's using them again," McCain said.

Ed Royce, chairman of the House committee, said if confirmed, the Damascus attack would mark a "significant escalation" in state-sponsored violence against civilians.

"This crisis poses a threat to US national security interests in the region."

Obama has deplored the violence which has killed more than 100,000 people in Syria, and demanded that Assad leave power.

But he is reluctant though to embroil US forces in a new war in the Middle East, after bringing American soldiers home from Iraq and as he ends the Afghan war.

The White House on Wednesday maintained stepped-up US aid to rebels did amount to "consequences" promised by Obama for chemical arms use.

But Earnest admitted US policy had not "achieved our ultimate goal" -- the removal of Assad.

He said Washington did not yet have corroborative evidence that a new chemical attack had happened.

"If the Syrian government has nothing to hide and is truly committed to an impartial and credible investigation of chemical weapons use in Syria, it will facilitate the UN team's immediate and unfettered access to this site," he said.

The Syrian government denied the allegations.

Dempsey, chairman of the Joint Chiefs of Staff, said in a letter to Engel that top military leaders were not "reticent, weary or risk averse" but rather mindful of the costs of war and "pragmatic about the limits of military force" in Syria.

He said knocking out Assad's air force was an option but it could drag the United States into an open-ended war.