But the Patriots and F-16s would have some utility if the United States decided to support the establishment of a buffer zone between Syria and Jordan. Contingency plans for such a zone, which would be enforced by Jordanian troops on the Syrian side of the border and supported politically by the United States, have already been developed.

The Obama administration is also trying to add to the diplomatic pressure on the Assad government. Mr. Kerry, in a phone conversation on Saturday with Iraq’s foreign minister, Hoshyar Zebari, called on Iraq “to take every possible measure to help end the military resupply of the Assad regime” and thus increase pressure for a political settlement, according to a statement released by the State Department.

During a March visit to Baghdad, Mr. Kerry pressed the Iraqi prime minister, Nuri Kamal al-Maliki, to inspect Iranian flights that American intelligence says are carrying weapons, supplies and personnel to Damascus to support the Assad government. There was a lull in Iranian flights after Mr. Kerry’s visit, but the flights resumed in May.

Although Mr. Kerry told Mr. Zebari that the United States was still seeking a political solution for the Syrian conflict, he also said that the Assad government’s use of chemical weapons and the Lebanese group Hezbollah’s decision to join the fight on the side of the Syrian government “threatens to put a political settlement out of reach,” the State Department said.

Even as the United States sought to build up the pressure on Mr. Assad, however, Mr. Kerry’s Russian counterpart pushed back, condemning the White House decision to send arms to the Syrian opposition and challenging its assertions that Mr. Assad’s forces had used chemical weapons.

At a news conference in Moscow, Sergey Lavrov, the Russian foreign minister, said that the evidence the Obama administration had relied on in making its charges of chemical weapons use was unreliable because the samples were not properly monitored until they reached a laboratory.

“There are rules of the Organization for the Prohibition of Chemical Weapons, which are based on the fact that samples of blood, urine, soil, clothing are considered serious proof only if the samples were taken by experts, and if these experts controlled these samples all the time while they are transported to a proper laboratory,” he said.