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The U.S. could hit Syria with three days of missile strikes, perhaps beginning Thursday, in an attack meant more to send a message to Syrian President Bashar al-Assad than to topple him or cripple his military, senior U.S. officials told NBC News on Tuesday.

The State Department fed the growing drumbeat around the world for a military response to Syria's suspected use of chemical weapons against rebels Aug. 21 near Damascus, saying that while the U.S. intelligence community would release a formal assessment within the week, it was already "crystal clear" that Assad's government was responsible.

Vice President Joe Biden went even further, bluntly telling an American Legion audience in Houston: "Chemical weapons have been used."

"No one doubts that innocent men, women and children have been the victims of chemical weapons attacks in Syria, and there's no doubt who's responsible for this heinous use of chemical weapons in Syria: the Syrian regime," Biden said.

White House press secretary Jay Carney repeated Tuesday that the White House isn't considering the deliberate overthrow of Assad.

"The options that we are considering are not about regime change," said during a daily briefing with reporters. "They are about responding to the clear violation of an international standard that prohibits the use of chemical weapons."

But Sen. John McCain, R-Ariz., an influential voice on military matters, pressed the administration to go further, calling for the U.S. and its allies to provide weapons to "the resistance on the ground."

"The important part of this whole situation is, is this just going to be just a retaliatory strike that has no lasting impact or something that changes the momentum on the ground in Syria?" McCain told reporters in Mesa, Ariz., after an event on immigration reform.

Three days of airstrikes planned

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Senior officials told NBC News that Defense Department planning had advanced to the point that three days of strikes were anticipated, after which strategists could run an assessment and target what was missed in further rounds.

U.S. missile strikes would almost certainly be launched from Navy destroyers or submarines in the Mediterranean Sea. The U.S. in recent days has moved destroyers closer to Syria, which sits on the sea's eastern edge, but that was mostly a symbolic move. U.S. Tomahawk missiles are so precise that they can hit not just buildings but also specific windows, and they could hit Syrian targets from far farther west in the Mediterranean.

Navy officials said four destroyers are lined up ready to strike: the USS Barry, the USS Mahan, the USS Ramage and the USS Gravely.

Tuesday, a fifth guided-missile destroyer, the USS Stout, also entered the Mediterranean, through the Straights of Gibraltar, but officials said it wouldn't take part in any cruise missile attack.

"The four destroyers now in place have more than enough cruise missiles," one official said.

Pressure for a response builds

Underscoring the urgency facing world leaders, British Prime Minister David Cameron called Parliament back from vacation and said it would vote on action Thursday, and U.S. Defense Secretary Chuck Hagel said the U.S. military was "ready to go."

Monday, using forceful language, Secretary of State John Kerry said Syrian chemical attacks were a "moral obscenity" and accused the Assad regime not just of having used chemical agents but also of having covered up the evidence.

On Tuesday, the U.N. said its investigating team in Syria would delay its next outing by a day, to Wednesday. The team came under fire from unidentified snipers Monday on its way to check out the site of a suspected chemical attack near Damascus, the capital.

In Cairo, the Arab League said it held Assad responsible for the suspected attack. Qatar, Saudi Arabia and other countries condemned the use of unconventional weapons.

Support from the Arab League, even if limited, would provide crucial diplomatic cover for a Western strike on Syria. Action through the U.N. is unlikely because Russia, which supports the Assad regime, has a veto in the Security Council.

Some U.S. allies, notably Britain, have signaled that a limited strike could take place without Security Council approval. But Russian Foreign Minister Sergei Lavrov said it would be a "very grave violation of international law," and China said through its government-run news service that the U.S. must refrain from "hasty armed intervention."

In Syria, the top general in the Free Syria Army, the umbrella group comprising rebel factions, told NBC News' Richard Engel that airstrikes were necessary to stop Assad from launching even broader chemical attacks.

"If there is no action, we are afraid that in the coming days, not coming weeks, Bashar will use chemical weapons and chemical materials against very wide areas and, I'm afraid, to kill maybe 20,000 or 30,000 more people," he said.

Eric Baculinao, Baruch Ben-Chorin, Catherine Chomiak, Carrie Dann, Alastair Jamieson, Stacey Klein, Andrea Mitchell, Ron Simeone and Winstone Wilde of NBC News; Reuters; and The Associated Press contributed to this report.

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