U.S. President Barack Obama's administration said Tuesday it did not favor arming Syria's opposition but did not rule out the idea completely amid deadly clashes with Bashar al-Assad's regime.

Senator John McCain, a senior Republican and Obama's rival in the last presidential election in 2008, made a new call Monday on a visit to the region for Syria's rebels to be given weapons to "defend themselves."

A top U.S. security official was due to meet Israeli Prime Minister Benjamin Netanyahu on Sunday amid rising concerns over Iran and ahead of a trip by the Israeli premier to Washington.

The White House has said that National Security Advisor Tom Donilon would talk to senior Israeli officials on a range of issues, including Syria. An Israeli official said he would meet Netanyahu on Sunday afternoon.

The United States Wednesday dismissed Syria's call for a referendum on a new constitution, which critics see as a move designed to ease global outrage over the government's bloody crackdown.

"It's actually quite laughable -- it makes a mockery of the Syrian revolution," White House spokesman Jay Carney told reporters aboard Air Force One.

The United States on Tuesday voiced skepticism of promises by Syrian President Bashar al-Assad to Russia's foreign minister and said that he should instead immediately end the violence.

State Department spokeswoman Victoria Nuland said the United States would withhold judgment on Foreign Minister Sergei Lavrov's visit to Damascus but criticized Assad for making new pledges of domestic reforms.

A U.S. judge Tuesday set an October 22 trial date for an Iranian-American accused of plotting with senior Tehran officials to hire Mexican gangsters to kill the Saudi ambassador to Washington.

Federal Judge John Keenan rejected a request for more time by the defense lawyer for Manssor Arbabsiar, a used car salesman from Texas with dual U.S.-Iranian citizenship.

Senior Republican senator John McCain urged the United States Tuesday to consider arming the opposition fighting the forces of Syria's President Bashar al-Assad.

"We should start considering all options, including arming the opposition. The blood-letting has got to stop," he told reporters.

The United States closed its embassy in Syria and pulled out all its staff on Monday amid growing security concerns as President Bashar al-Assad's government intensifies its bloody crackdown.

Britain also recalled its envoy, but U.S. President Barack Obama stressed the importance of diplomacy and said it was a very different situation from Libya, where Western military intervention helped oust Moammar Gadhafi.

U.S. Secretary of State Hillary Clinton vowed Sunday to bolster existing sanctions against the Syrian regime and seek further ones to block funding and arms shipments to Damascus.

Seeking other ways to turn the screws on Assad a day after Russia and China vetoed a resolution on Syria, Clinton said Washington will also work with Syria's friends worldwide to support the peaceful aims of the opposition.

U.S. Secretary of State Hillary Clinton on Friday held "constructive" talks with her Russian counterpart Sergei Lavrov over a draft U.N. Security Council resolution condemning Syria's crackdown, officials said.

The pair agreed that diplomatic teams from both countries would "continue to consult" on the draft in New York, said State Department spokesman Mark Toner, amid a U.S. push for passage of the resolution.

The U.S. military plans to send a large floating base for commando teams to the Middle East amid rising tensions with Iran and intensifying fighting in Yemen, The Washington Post reported Saturday.

Citing unspecified procurement documents, the newspaper said the Navy is converting an aging warship it had planned to decommission into a makeshift staging base for the commandos in response to requests from the U.S. Central Command.