UN experts left Syria on Saturday after investigating a poison gas attack that killed hundreds of civilians, and the United States said it was planning a limited response to punish Syria's President Bashar al-Assad for the "brutal and flagrant" assault.
US President Barack Obama said the United States, which has five cruise-missile equipped destroyers in the region, was in the planning process for a "limited, narrow" military action that would not involve boots on the ground or be open-ended.
In a sign the United States may be preparing to act, US Secretary of State John Kerry spoke on Friday to the foreign ministers of key European and Gulf allies, as well as the head of the Arab League, a senior State Department official said.
A witness said the team of UN experts arrived at Beirut International Airport on Saturday, after crossing the land border from Syria into Lebanon by foot earlier in the day.
The 20-member team, including experts from the Organisation for the Prohibition of Chemical Weapons, have been into the rebel-held areas in the Ghouta suburb of Damascus three times, taking blood and tissue samples from victims. They also took samples of soil, clothing and rocket fragments.
They will be sent to laboratories in Europe, most likely Sweden or Finland, for analysis. The UN experts have already been testing for sarin, mustard gas or other toxic agents.
The analysis should establish if a chemical attack took place but not who was responsible for the Aug. 21 attack on a Damascus suburb.
Final results might not be ready for two weeks, UN Secretary-General Ban Ki-moon told Security Council members, according to diplomats.
The United States released its own unclassified intelligence report on the attack, which Kerry said killed 1,429 Syrian civilians and was clearly the work of Assad's forces.
"If we choose to live in the world where a thug and a murderer like Bashar al-Assad can gas thousands of his own people with impunity" it would set a bad example for others, such as Iran, Hezbollah and North Korea, Kerry said.
Syria blames rebels fighting to topple Assad for the attack . Its main ally Russia, which has