Obama on Syria: 'I have not made a decision'

Russia blocks action against Syria at UN 2:53

President Barack Obama made the case Wednesday for a strike against Syria to send a message and discourage the future use of chemical weapons — but said he had not yet made a decision on whether to take action.

“We have not yet made a decision but the international norm against the use of chemical weapons needs to be kept in place,” Obama said in an interview with PBS Newshour.

"If we are saying in a clear and decisive but very limited way, we send a shot across the bow saying, 'Stop doing this,' this can have a positive impact on our national security over the long term," he added.

The United States has been debating how to respond to an attack last week in the suburbs of Damascus that killed hundreds of people – an attack Obama said was carried out by the government.

What happens if the US strikes Syria? 2:46

“Nobody disputes -- or hardly anybody disputes -- that chemical weapons were used on a large scale in Syria against civilian populations,” the president said. “We have looked at all the evidence, and we do not believe the opposition possessed … chemical weapons of that sort.”

But Obama’s taking time to mull the U.S. options for action in Syria is not the only issue causing the delay of a potential strike.

Britain — a key ally for any action on Syria — changed its stance Wednesday, saying the U.N. Security Council should report findings from its inspectors before any military action is taken, and that the British parliament should vote on the matter twice.

Meanwhile, following a meeting of a divided U.N. Security Council Wednesday over the crisis, U.S. officials said the body won’t be able to reach consensus on Syrian action because the Russian delegation continues to block any resolution.

“This issue is dead. The Russians won’t budge,” one U.S. official told NBC News.

The five countries with permanent seats on the council held the preliminary meeting at the United Nations to discuss a resolution drafted by Britain that would condemn the use of chemicals and authorize “necessary measures” to protect Syrian civilians.

Russia and China, which have veto power, have warned the United States not to bypass the U.N. and strike Syria. Secretary-General Ban Ki-moon said U.N. inspectors need more time to assess whether the forces of Syrian leader Bashar Assad used poison gas on civilians.

The U.S. administration's view has been that the U.N. investigation comes too late to produce credible results.

"We've consistently said that we support U.N. Security Council action. My understanding is that today we heard nothing different from the Russians in today's meeting than we have for months and indeed years about Syria,” said State Department spokeswoman Marie Harf.

Harf signaled that the U.S. will not wait for U.N. action. "We cannot be held up in responding by Russia's intransigence -- continued intransigence at the United Nations. That, quite frankly, the situation is so serious that it demands a response," she said.

China has also said military action would not help solve the crisis in Syria, but also repeated that it opposed anyone using chemical weapons. 

"A political resolution has, from the very beginning, been the only way out for the Syrian issue," Chinese Foreign Minister Wang Yi said in a statement on the ministry's website. 

Shaping up as another obstacle, a bipartisan group of 116 Congressmen (98 Republicans and 18 Democrats) signed a letter to Obama asking him "to consult and receive authorization from Congress before ordering the use of U.S. military force in Syria."

"Engaging our military in Syria when no direct threat to the United States and without prior Congressional authorization would violate the separation of powers that is clearly delineated in the Constitution," the letter read.

House Speaker John Boehner also called on Obama to deliver a specific rationale for using U.S. military force against Syria.

Pentagon officials told NBC News that Navy destroyers, plus two American and one British submarine, were in position in the Mediterranean Sea and ready to launch Tomahawk missiles if Obama gives the order.

Exclusive video: Apparent chemical attack aftermath 1:46

A fifth Navy destroyer has entered the Mediterranean on a scheduled swap with one of the four other destroyers but is not expected to take part in the missile strikes.

At the U.N., Syria’s ambassador, Bashar Ja’afari, said flatly: “We know that the Syrian government is totally innocent of these accusations.” He also said that Syrian rebels had unleashed poison gas on regime forces and asked the U.N. to investigate.

He insisted that U.N. inspectors be allowed to work without political or military pressure. He would not confirm reports that Syria was already evacuating military bases and government buildings, but he said: “We are in a state of war right now.”

Activists said Thursday morning that the regime had begun shelling Eastern Ghouta, the area where the U.N. inspectors were investigating.

Experts say U.N. inspectors will scour the soil for evidence of nerve agents as they investigate last week's alleged chemical attack.

Craters where munitions exploded and shell fragments could provide clues as to who was behind the incident. The meticulous evidence-gathering process is likely to be filmed and exact locations where samples were found will be recorded using GPS technology.

A "chain of custody" will ensure the samples cannot be tampered with. Several witnesses will watch as evidence is placed in sealed containers. They will not be re-opened until they reach the laboratory. Samples are likely to be sent to several sites in the U.S., U.K., Finland, Netherlands and Switzerland. 

Prof. Alastair Hay, a U.K.-based chemical weapons expert who has conducted six investigations into alleged incidents, said that once the evidence has been gathered the testing process could take anywhere from a few days to several weeks. That raises the possibility that the U.S. and its allies could strike at Syrian targets before the U.N. has concluded its inquiry.

NBC News' Andrea Mitchell, Charlene Gubash, Catherine Chomiak, Alastair Jamieson, Erin McClam and Becky Bratu contributed to this report. Reuters also contributed.