U.S. Secretary of State John Kerry said on Monday the chemical weapons attack on civilians in Syria is "inexcusable" and "undeniable" despite "the excuses and equivocations that some have manufactured."

U.S. Secretary of State John Kerry(R)

"Indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity," Kerry said in at a press briefing.

"What is before us today is real, and it is compelling," said Kerry.

"The reported number of victims, the reported symptoms of those who were killed or injured, the firsthand accounts from humanitarian organizations on the ground like Doctors Without Borders and the Syria Human Rights Commission – these all strongly indicate that everything these images are already screaming at us is real, that chemical weapons were used in Syria," he said.

"We know that the Syrian regime maintains custody of these chemical weapons," Kerry said, added that "We know that the regime has been determined to clear the opposition from those very places where the attacks took place."

He said additional information about the chemical weapons attack will be provided later.

Kerry accused the Syrian government's attempt to cover up the evidence of chemical weapons attack.

"For five days, the Syrian regime refused to allow the UN investigators access to the site of the attack that would allegedly exonerate them," Kerry said.

"It attacked the area further, shelling it and systematically destroying evidence. That is not the behavior of a government that has nothing to hide. That is not the action of a regime eager to prove to the world that it had not used chemical weapons," Kerry continued

He believed that Syrian government's decision to allow UN access is "too late to be credible".

Kerry also said that the reported attack on the UN investigators and continued shelling of these very neighborhoods "further weakens the regime's credibility."

He said the U.S. administration has been "reviewing the situation in Syria" and is weighing how to respond by consulting with U.S. allies and Congress members.

The latest allegations of chemical weapons use concern the Ghouta area outside Damascus. The attack, on 21 August, reportedly killed more than 300 civilians including children.

Since fighting began in March 2011 between the Syrian Government and opposition groups seeking to oust President Bashar Al-Assad as many as 100,000 people have been killed, including more than 7,000 children.

In addition, almost 2 million have fled to neighbouring countries and a further 4 million have been internally displaced. At least 6.8 million Syrian require urgent humanitarian assistance, half of whom are children.