DAMASCUS, Syria — The United Nations says that a team of chemical weapons experts in Syria has delayed a second trip to investigate an alleged poison gas attack near Damascus by one day.

The United Nations says in a statement the decision was made Tuesday in order to improve preparedness and safety for the team, after unidentified snipers opened fire on their convoy on Monday.

Syrian Foreign Minister Walid al-Moallem said at a press conference earlier that Tuesday's trip was postponed because of disputes between rebel groups.

The U.N. statement only mentioned security precautions.

It urged all sides in the conflict to give safe passage and access to the team, adding it was in the interest of all sides to cooperate with the investigation.

The Washington Post cited senior administration officials as saying President Barack Obama was weighing limited military strikes on targets in Syria.

Such action would probably last no more than two days and involve cruise missiles, or possibly long-range bombers, striking military targets not directly related to Syria's chemical weapons arsenal, the newspaper said.

Russia, the Damascus regime's most powerful ally, warned any use of force against Syria would have "catastrophic consequences" while calling on the United States to show "prudence" and adhere to international law.

It also voiced regret that Washington had scrapped a planned meeting with it this week on the Syria crisis.

In Asia, stocks were mostly down and oil prices were up — shifts blamed on fears of further escalation in the brutal 29-month-old conflict, this time due to the possibility of American intervention that Obama has steadfastly tried to avoid.

Secretary of State John Kerry accused President Bashar al-Assad's regime of a cover-up, but said Washington would provide more evidence of who was behind the attacks.

"Let me be clear. The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity," said Kerry. "We have additional information about this attack, and that information is being compiled and reviewed together with our partners, and we will provide that information in the days ahead.

"Make no mistake. President Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people. Nothing today is more serious," Kerry said.

Kerry spoke after U.N. inspectors met survivors of the attacks, which the independent medical agency Doctors Without Borders has said left at least 355 people dead from "neurotoxic symptoms."

The U.N. convoy came under sniper fire on Monday as it tried to approach the Damascus suburb where one of the attacks was reported, but the team managed to visit victims receiving treatment in two nearby hospitals.

"It was a very productive day," U.N. spokesman Farhan Haq told reporters, adding that the team, led by Swedish expert Aake Sellstroem, was "already gathering valuable evidence."

U.N. Secretary General Ban Ki-moon said despite the "very dangerous circumstances" the investigators "interviewed witnesses, survivors and doctors" and "collected some samples."

The U.N. team was in a buffer zone between government and opposition-held areas when it came under attack.

Ban said the United Nations had made a "strong complaint" to the Syrian government and opposition forces, which traded blame for the sniper fire just as they did the chemical attacks.

The United States accused government forces of resuming their shelling of the attack site soon after the U.N. team departed in a bid to destroy evidence.

President Vladimir Putin of Russia — which provides Syria's regime with diplomatic cover by blocking Security Council action — was unimpressed by the mounting evidence of an atrocity.

Putin on Monday told British Prime Minister David Cameron there was no proof Damascus had used chemical weapons, according to Cameron's office.

Cameron cut short his holiday on Monday to return to London to plan a response. Britain, along with France, has been in the forefront of demands for tougher action against Assad.

Senior military officers from Western and Muslim countries began gathering in Jordan Monday to discuss the regional impact of the war in Syria, Jordanian officials said.

U.S. army chief General Martin Dempsey will take part, as would chiefs of staff from Saudi Arabia, Qatar, Turkey, Britain, France, Germany, Italy, and Canada, said an official, cited by state news agency Petra.

A senior Israeli delegation meanwhile visited the White House for high-level talks on the Syrian crisis and the showdown over Iran's controversial nuclear program.

The Syrian opposition says more than 1,300 people died when toxic gases were unleashed on two neighborhoods on the outskirts of Damascus.

Syria approved the U.N. inspection on Sunday, but US officials said it was too little, too late, arguing that persistent shelling had "corrupted" the site.

With China and Moscow expected to boycott any resolution backing a military strike, British Foreign Secretary William Hague said the West could act even without full U.N. Security Council backing.

There is also precedent for Obama to act militarily without congressional backing, despite a law technically requiring it.

The alleged gas attacks are only the latest atrocity in a conflict that has claimed more than 100,000 lives since March 2011.

Assad, in an interview with a Russian newspaper published Monday, denied accusations his government was behind the attacks, calling the charges an "insult to common sense."