Obama advocates 'shot across the bow' for Syria, as concerns grow in Congress

President Obama on Wednesday advocated a “shot across the bow” for Syria in the interest of U.S. national security, despite growing concerns from congressional lawmakers over the possibility of an American military strike.

In an interview with PBS, Obama for the first time said publicly the U.S. has concluded the Syrian government carried out a chemical weapons attack against civilians last week, saying the administration does not believe the country’s opposition has such weapons at their disposal.

Obama’s comments came shortly after the White House announced it will meet with top congressional leaders Thursday afternoon to brief them on intelligence and plans for possible strikes against Syria.

Meanwhile, in potentially related news, a senior U.S. Navy official confirmed to Fox News Wednesday that the Navy is beefing up its presence in the Gulf, increasing the number of aircraft carriers to two, from one.

The USS Harry S Truman has arrived in the Arabian Sea and was scheduled to take the place of the USS Nimitz, which was expected to head home. The Navy has ordered the Nimitz, which is in the Indian Ocean, to stay for now.

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On Thursday, the Russian news agency Interfax, citing a source in that country's armed forces, reported that Russia would send an anti-submarine ship and missile cruiser to the Mediterranean.

Obama said Wednesday he has not yet made a decision on how to respond, but said “international norms” state the use of chemical weapons should not be tolerated.

“We cannot see a breach of the nonproliferation norm that allows, potentially, chemical weapons to fall into the hands of all kinds of folks,” Obama said, saying U.S. national security could be at risk if that occurred.

Citing Americans’ concern over getting involved in another long conflict like the Iraq War, Obama said the U.S. can instead choose “limited, tailored approaches” to military involvement in Syria.

“If we are saying in a clear and decisive but very limited way, we send a shot across the bow saying, ‘stop doing this,’ that can have a positive impact on our national security over the long term,” Obama said.

A bipartisan group of 116 members of Congress sent Obama a letter Wednesday urging the administration to seek Congress’ approval before taking any action, saying to do otherwise would be unconstitutional.

Several lawmakers have expressed concern over the possibility of the U.S. intervening in the conflict, with one senior aide telling Fox News there are fears launching missiles at Syria could be a case of "fire and forget."

Additionally, lawmakers want to know what the endgame is with Syria, why the U.S. is acting now and what it is expecting as an outcome.

"We don't employ the US military just to make a point," one congressional source, who asked not to be identified, tells Fox News.

House Speaker John Boehner also sent a letter to the White House Wednesday, asking Obama to "personally make the case to the American people and Congress for how potential military action will secure American national security interests, preserve American credibility, deter the future use of chemical weapons, and, critically, be a part of our broader policy and strategy."

The White House has not publicly commented on Boehner’s request.

Fox News' Chad Pergram and Jennifer Griffin contributed to this report