MOSCOW -- The Russian government on Saturday stepped up its attack on the accusation by the United States that Syria had used chemical weapons against the rebels in its civil war, saying that evidence cited by the Americans was unreliable because the samples were not properly monitored until they reached a laboratory.
The angry criticism by Foreign Minister Sergey V. Lavrov at a news conference in Moscow was a setback to the United States' efforts to forge a common position with the Kremlin on how to end the conflict, which has killed more than 90,000 Syrians.
In a tangible move to defend against a potential Syrian chemical weapons attack and also reassure an important ally, Defense Secretary Chuck Hagel has decided to increase the Pentagon's presence in the region by approving a request from Jordan for the deployment of American warplanes and antimissile batteries there, officials said Saturday.
George Little, the Pentagon press secretary, issued a statement saying that a detachment of American F-16 warplanes as well as Patriot missile-defense systems would remain in Jordan after the end of a military exercise next week. All other American personnel assigned to the exercise will depart when it ends, he said.
"The United States enjoys a longstanding partnership with Jordan and is committed to its defense," Mr. Little said.
The moves came as Secretary of State John Kerry delivered a stark warning on Saturday that the Syrian government's use of chemical weapons and Hezbollah's decision to join the fight may make it impossible to achieve a political settlement.
In a telephone conversation with his Iraqi counterpart, Mr. Kerry stressed that the United States still seeks a political solution to the Syrian conflict, but added pointedly that "the use of chemical weapons and increasing involvement of Hezbollah demonstrates the regime's lack of commitment to negotiations and threatens to put a political settlement out of reach," according to a statement issued by the State Department.
Mr. Kerry flew to Moscow last month and secured Russia's agreement to hold an international conference to try to bring an end to the fighting. Since then, however, the Obama administration has become concerned that the advances made by pro-government forces would give Syria's president, Bashar al-Assad, little reason to negotiate a political transition in which he would give up power.
As a result, White House and State Department officials now favor delaying the talks. The provision of arms to the rebels, officials have said, is partly intended to turn the tide enough to force a real negotiation.
Another complication for any diplomatic effort is strains in the American and Russian relationship over Syria, which have grown since the Obama administration said it was convinced the Assad government had used chemical weapons.
Russia has long supported the Assad government politically and by sending arms. Russia has also supported its refusal to agree to wide-ranging international inspections of potential chemical weapons use.
A major concern of the Russians is that allegations of chemical weapons use will become a rationale for greater American and Western involvement in the crisis, including possible military action.
Mr. Lavrov, speaking at a news conference on Saturday after meeting with Italy's foreign minister, Emma Bonino, urged the United States to adhere to a plan to hold an international conference on Syria that would bring the Assad government and the rebels to the negotiating table.
In addition to questioning the reliability of the evidence, Mr. Lavrov complained that the suggestion of arms shipments by the United States would only prolong the bloodshed by encouraging the rebels to keep fighting.
"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," Mr. Lavrov said, "and if these experts controlled these samples all the time while they are transported to a proper laboratory."
Mr. Lavrov said that the use of chemical arms made no sense, given the current state of the conflict. "The regime has not been driven into a corner now," Mr. Lavrov said. "What sense does it make for the regime to use chemical weapons, especially in such small quantities, only to expose itself?"
The White House referred questions on Mr. Lavrov's charges of faulty intelligence to the director of national intelligence's office.
On Thursday, Ben Rhodes, a deputy national security adviser, told reporters in a conference call that chemical weapons were used "on a small scale against the opposition multiple times in the last year."
Mr. Rhodes and other American officials said that the assessment is based on evidence that includes physiological samples, intelligence on the Assad government's plans for the use of chemical weapons, accounts of specific attacks and descriptions of symptoms experienced by victims of the attacks.
One classified C.I.A. report from mid-March states that blood, hair and urine samples were taken from two rebel fighters -- one dead and one wounded -- who were exposed to sarin gas in a town near Damascus.
In Washington, Mr. Kerry sought again to elicit Iraq's cooperation in tamping down the conflict in Syria. In a phone call to Hoshyar Zebari, the Iraqi foreign minister, Mr. Kerry urged the Iraqi government to discourage Iraqi Shiites from joining the conflict. A substantial number of Iraqi Shiites from militias like Kataib Hezbollah and Asaib al-Haq have joined the fighting in Syria on the government's side, with the encouragement of Iran, according to American intelligence.
Mr. Kerry, the State Department said, also "urged that Iraq take every possible measure to help end the military resupply of the Assad regime." Mr. Kerry pressed Prime Minister Nuri Kamal al-Maliki of Iraq to inspect the Iraqi flights during a March visit to Baghdad. There was a lull in Iranian flights after Mr. Kerry's visit, but the flights resumed again in early May and are believed to be carrying weapons and war matériel for the Assad government.
The United States' conclusions on the use of chemical weapons have been echoed by its main allies.
David M. Herszenhorn reported from Moscow, and Michael R. Gordon from Washington. Thom Shanker and David E. Sanger contributed reporting from Washington.world
This article originally appeared in The New York Times.