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But backing away from a “red line” would hurt President Obama’s credibility, said Danielle Pletka, vice president for foreign and defense policy for the American Enterprise Institute.

“When you’re in a position of redefining your rhetoric, you’re in a bad place. It means not only do you not have a policy, you don’t have a clear set of principles that define your vision for the American role in the world,” she said.

The United States plans to double its nonlethal assistance to opposition fighters, bringing the total to about $250 million in aid since the fighting began, Secretary of State John F. Kerry announced Sunday.

The newest round of aid may include for the first time armored vehicles, body armor, night-vision goggles and other military supplies.

The United States has supported the supplying of arms from allies and has deployed Patriot missiles along Turkey’s border with Syria. Last week, Washington announced the deployment of 200 U.S. soldiers to Jordan to assist efforts to contain violence along that country’s border.

The United States also will provide an additional $25 million in food assistance for Syrian residents and refugees, bringing the total food and medical aid to $409 million.

“We appreciate the limited support given by the international community, but it is not sufficient,” the opposition Syrian National Coalition said in a statement released at an international conference Sunday in Istanbul.

“We call on the international community to be more forthcoming and unreserved to fulfill its responsibilities in extending support that is needed by the Syrian people.”

Some European Union members, notably Germany and the Netherlands, fear that more weapons flowing into Syria will only increase the bloodshed and that the arms could fall into the hands of extremists.

“We need to figure out who in the opposition we want to help, and then we need to help them,” Ms. Pletka said. “This is not an administration that believes in being in front on anything.”

This story is based in part on wire service reports.