Syrian President Bashar al-Assad’s forces are “gaining physical momentum on the battlefield,” and the situation there “will get worse before it gets better,” the top U.S. military official in the region said Tuesday.

Marine Gen. James Mattis told Senate lawmakers that Assad “is going to be there for some time because I think he will continue to employ heavier and heavier weapons on his people.”

Any U.S. or international air operations against Assad’s forces would be “challenging,” said Mattis, head of U.S. Central Command. Russia has provided Syria with “very advanced integrated air defense capabilities — missiles, radars, that sort of thing,” he said.

Iranian support for Assad has included weapons and teams of experts that have flown into Damascus to provide intelligence and eavesdropping capabilities to locate and suppress opposition networks, Mattis said, adding that Iran has also been “moving weapons” into Sudan and to opposition factions in Yemen.

President Obama was more direct in saying that U.S. military intervention in Syria, proposed by Sen. John McCain (R-Ariz.) and others, would be “a mistake.”

Obama called the situation in Syria “heartbreaking and outrageous” and said the United States would continue working with the international community to support and unify the Syrian opposition, provide humanitarian assistance and put political and economic pressure on Assad.

“The notion that the way to solve every one of these problems is to deploy our military, you know, that hasn’t been true in the past and it won’t be true now,” Obama said at a news conference that focused on administration policy toward Iran.

“We’ve got to think through what we do through the lens of what’s going to be effective,” he said, “but also what’s critical for U.S. security interests.”

McCain’s call to arms, which he made in a speech on the Senate floor Monday and repeated at a Tuesday hearing of the Senate Armed Services Committee, gained little support even among Republicans. Sens. Lindsey O. Graham (R-S.C.) and Joseph I. Lieberman (I-Conn.) signed on, but House Speaker John A. Boehner (R-Ohio) called it “premature” and said, “I think the situation in Syria is pretty complicated.”

Mattis, who testified before the committee along with Adm. William H. McRaven, head of the Special Operations Command, said he had not been directed to do any detailed planning on any U.S. operations in Syria, including providing training for opposition military forces.

The terrain in the northern part of the country where McCain proposed to establish “safe havens” for the opposition has no natural barriers to forestall a government attack, unlike the mountainous region of northern Iraq where Kurds held out against Saddam Hussein.

“It would be a significant commitment of resources,” Mattis said.

He said the United States was “watching very closely” Syria’s stockpiles of chemical weapons, among the largest in the world, but had not seen any effort by Assad to use them.

If the weapons were left unsecured, presumably in the event of the government’s fall, “it would be, potentially, a very serious threat in the hands” of militant groups such as Lebanon’s Hezbollah, he said.

Administration officials have previously said that al-Qaeda forces from Iraq were active in Syria, and Mattis said they had carried out a series of “rather spectacular” attacks using improvised explosive devices. But while they are taking advantage of the “chaos” in Syria, he said, al-Qaeda infiltrators “do not define the opposition to Assad.”

At the United Nations, U.S. Ambassador Susan E. Rice said that “preliminary discussions” had begun among the permanent five members of the Security Council and Morocco, which sponsored a resolution on Syria vetoed last month by Russia and China, on a new resolution “that would demand an end to the violence in Syria and demand immediate humanitarian access.”

U.S. and European officials have expressed hope that Russia, following what they called the “distraction” of last weekend’s presidential election, would be more amenable to joining international efforts to stop Assad’s bloody onslaught against Syrian civilians in opposition strongholds.

But on Tuesday, Russia warned against what it called “wishful thinking.”

“Russia’s stance on the Syrian settlement has never been subject to any short-term considerations, and hasn’t formed under the influence of electoral cycles, unlike that of some of our Western colleagues,” a Foreign Ministry statement said.

The vetoed resolution called for all violence to stop; a new draft circulating as of Monday night at the United Nations appeared likely to be even less palatable to the Russians. It calls for the opposition to “refrain from all violence,” but only after government forces cease firing and withdraw from all cities and towns.

While it does not directly call for Assad to step aside, the draft supports U.N. and Arab League efforts “in facilitating a Syrian-led political transition” and a political dialogue between the government and opposition.

The draft calls for the Security Council to review implementation of its demands two weeks after passage and, “in the event of noncompliance, to consider swiftly further measures.”

Staff writers Colum Lynch and Ed O’Keefe contributed to this report.